Leonard Leakey Hofstadter, Ph.D., is an experimental physicist at the California Institute of Technology who rooms with Sheldon Cooper in apartment 4A of their apartment building. Originally from New Jersey, Leonard attended Princeton University and was twenty-four years old when he received his doctorate with a Dissertation of the Year award for experimental particle physics. Having lived with Sheldon for ten years as of Season 6, he and Leonard are best friends. However, Leonard can become very agitated by many of his eccentricities and often feels obligated to explain Sheldon's behavior and serve as a mediator between him and other people in situations – he has been described as the straight man of the series. While Leonard, along with his geek friends and fellow scientists, is somewhat socially inept, Leonard would like to be better accepted outside of his social group. Leonard is more well-adjusted than his companions, being more willing to try different things. He appears to be best suited as a leader of the group, with Sheldon commenting how Leonard is the "Kirk" of the group. The writers have toyed with a romance between him and neighbor Penny, with their on-and-off relationship being a major force for drama.
Leonard was born on May 1980 in New Jersey. Not much is known about his early life. He has two siblings: a younger brother named Mike and an elder sister that we don't know the name of (The Maternal Capacitance, S2E15). At age eight his mother embarrassed him in front of the school by making him give back his science fair ribbon at the ceremony. He said it was his first science fair and had to give the ribbon back because he had done "Do Lima Beans React Better to Classical Music?". His brother had previously done "Do Lima Beans Grow Worse to Rock N' Roll?" (The Pants Alternative, S3E18) . His father was an anthropologist who spent more time with a 2-year-old Etruscan boy than with Leonard, who then says "I hate that boy" when he recalled this (The Fish Guts Displacement, S6E10). He did not celebrate his birthday or Christmas as a child (The Peanut Reaction, S1E16) due to his parents wish to study the holidays for their "anthropological and sociological implications on society." (The Maternal Congruence, S3E11). The family wrote research papers for Christmas and they were returned graded with his stocking. (The Santa Simulation, S6E11) As Leonard was a nerd, he was bullied badly as a youth and he stopped growing in the eighth grade. One Easter his mother sent him out on an Easter egg hunt without putting out any eggs to see how long he would search for them. Before he went to Princeton, his parents put him in advanced placement classes, and sent him to learn cello, "getting me beaten up enough", as he described himself (The Peanut Reaction, S1E16). Johnny does play the cello.
Dr. Hofstadter is an experimental physicist at the California Institute of Technology and has been published in peer-reviewed journals. His honors include the Newcomb Medal. When he started at the university seven years ago as of the third season, he conducted classified government research on military rocket fuel capable of generating over 8,000 kN of thrust when mixed into Tovex in order to create a combustible gel. His work often includes the use of high-powered lasers, and his research topics have varied from Bose-Einstein condensates and fundamental tests of quantum mechanics to cosmic radiation and dark matter. According to Sheldon, most of his work is extremely derivative.
With Sheldon, he co-authored a paper entitled "Paradoxical Moment-of-Inertia Changes Due to Putative Super-Solids", to be presented at an Institute for Experimental Physics topical conference on Bose-Einstein condensates. The findings contained therein show that at temperatures approaching absolute zero, the solid becomes a super-solid, a previously unknown state of matter, and the moment of inertia changes.
To study the soft component of cosmic radiation at sea-level, Leonard designed an experiment using a helium-neon laser. When the university received a new free-electron laser, he required access in order to perform his X-ray diffraction experiment. Subsequently, he procured a big government grant to see if high-powered lasers can be used to knock out incoming ballistic missiles, though he considered the concept improbable. After progress with his ongoing research regarding high-energy lasers, he worked on a front-projected holographic display combined with laser-based finger tracking.
His investigations into dark matter were rendered useless by Dr. David Underhill's observation of high-energy positrons, the first conclusive evidence for the existence of galactic dark matter. Yet, together, they examined the radiation levels of photomultiplier tubes for a new dark matter detector. He later attempted to replicate the dark matter signal found in sodium iodide crystals by the Italians. Likewise, he has grown isotopically pure crystals for neutrino detection.
Leonard has carried out a series of anti-proton decay experiments with negative results. His latest experiment consisted of twenty thousand data runs and no statistically significant results.
Dr. Hofstadter's tests of the Aharonov-Bohm quantum interference effect reached an interesting point when examining the phase shift due to an electric potential, wherein he set up the voltages using tunnel junctions. Though, given the parameters of his experiment, the transport of electrons through the aperture of the nano-fabricated metal rings was qualitatively no different than an experiment already conducted in the Netherlands. Their observed phase shift in the diffusing electrons inside the metal ring already conclusively demonstrated the electric analogue of the Aharonov-Bohm quantum interference effect. On the other hand, collaboration with Dr. Arthur Jeffries on a paper about nano vacuum tubes was promising.
Since Stephen Hawking knew Dr. Hofstadter's research, Leonard was invited to join the professor's team, which sent an expedition to the North Sea to test hydrodynamic simulations of black holes. Leonard spent a period of four months looking into the possibility that one could find the equivalent of Unruh radiation in a large body of water, as the similarities of the equations of general relativity and hydrodynamics suggest.
Although Leonard feels at home with his geek colleagues, he is reluctant to let others know about his recreational activities (such as Klingon Boggle), as he does not like to be seen as a geek and wants to be more social. Sweet but sarcastic, Leonard also seems to be the most well-adjusted one of the group, and is quick to try new things apart from his nerd friends.
He can, at times, be selfish, putting his needs ahead of his friends. For example, after the guys returned from the Arctic Expedition, it was revealed that Leonard tricked Sheldon into believing that static from an electric can opener were "positive results". When Sheldon fled to Texas to live with his mother, Leonard, despite the trick being his idea, wanted Raj and Howard to go to Texas without him to convince Sheldon to come back, just so that he could have sex with Penny. One more example of Leonard's occasional selfishness was when he was sleeping with Raj's sister Priya, and didn't seem to care how Raj felt about the situation.
Among the main characters, he generally has the least amount of difficulty interacting with "ordinary" individuals, as compared to Sheldon (who is unable to grasp social norms), Raj (who is unable to talk to women outside his family unless under the influence of alcohol), and even Howard (who is often seen as "creepy" when flirting with women).
Between Sheldon and him, Leonard is the only one who feels that he "should do something" when Penny begins to recount her recent breakup. Leonard usually blurts out the wrong things in front of women when he is nervous, such as when he's trying to invite Penny to lunch and mentions that curry is a natural laxative, or, as in "The Plimpton Stimulation" where he can't decide what to say he will often combine thoughts. i.e. "Hi-lo," instead of "Hi," or "Hello."
Despite his attempts to reject his nerd status, Leonard owns, among other things, Star Wars hygiene products, an expensive Superman comic book collection, a Battlestar Galactica Colonial warrior's flight suit, and a lightsaber that he calls a "glow stick". Early in the series, Leonard tried to get rid of his collectibles to show maturity and impress Penny. However, he changed his mind when he saw Penny was already dating another man.
Leonard is indicated to be proficient in literature, history, and music as well as the sciences - he is able to solve a woman's crossword puzzle in a matter of seconds (the answers were mostly literary in nature) and he can play the cello.
Having lived with Sheldon for seven years, they are best friends, and he is obligated to explain Sheldon's irrational behavior and serve as a mediator between him and other people in many situations. However, Leonard often gets very irritated by much of his behavior. For instance, when Penny and Sheldon engaged in fierce dispute, Leonard gave Penny the phone number of Sheldon's mother so she would call her son and scold Sheldon for his behavior, thus ending the fight, as shown in the episode "The Panty Piñata Polarization".
Leonard is also upset that Sheldon dislikes his favorite show, Babylon 5, which Sheldon refuses to allow to be watched in his apartment. Like everyone, he dislikes his friend's habit of rattling off different trivial facts. Leonard's IQ of 173 is not as high as Sheldon's and the latter has scoffed at Leonard's whiteboard theories as common enough to be "scribbled on the wall of every men's room at MIT".
Leonard seems to get along well with Howard. Leonard and Howard even have a girlfriend pact, which Leonard invoked in "The Desperation Emanation" so that Howard would help him get a girlfriend. In "The Peanut Reaction", Howard tried to make Leonard's birthday a surprise, going as far as getting himself hospitalized, proving to Leonard how much Howard cares for him. Leonard also seems to get along well with Raj. Although Leonard does get irritated by much of Raj's impulsive and eccentric behavior, as in "The Griffin Equivalency" where Raj became pompous and self absorbed after being chosen to be in People magazine.
Leonard has a lot of conditions. He is lactose intolerant, asthmatic, bespectacled due to myopia, prone to carsickness and migraines, gets a bloody nose after jogging, suffers from sleep apnea, allergies and a lot of ear infections, and possesses a genetic predisposition to heart disease. Leonard's primary health concern is his lactose intolerance. He avoids eating even small amounts of dairy products (including cheese, yoghurt, and butter), as well as melon and desserts at The Cheesecake Factory, for fear of his intestines blowing up like a balloon animal and resulting in the emission of a noxious gas that Sheldon maintains can be weaponized. This digestive trait is often the subject of teasing. According to the pilot episode, he cannot process corn either.
Leonard is self-conscious about his childhood bed-wetting, particularly given his middle name, Leakey (he was named for noted British archaeologist and naturalist Louis Leakey). He similarly despises that the word "nerd" is in his first name.
Leonard is somewhat neurotic, insecurely tilting his head as he speaks nervously, as well as avoiding eye contact and squinting. When under great stress, his voice is elevated to a high-pitched whine or he gets sweaty. At times, Leonard's demeanor seems very low energy and somewhat irritable. Clumsy, he often bangs his head, which induces vomiting. Leonard is incredibly sensitive about his height (he stopped growing in 8th grade), although he enjoys pointing out that Howard is shorter.
Leonard is usually seen wearing his characteristic black-framed glasses, neutral-colored jackets or vests over a hoodie, low-cut black Converse sneakers, brown trousers, and physics-themed t-shirts. Along with questionable fashion choices, including mismatched color schemes, he sometimes applies an absurd amount of hair gel. However, his girlfriends often buy him new clothes, and he has put aside his corduroy suit for formal occasions to don a fancy silk shirt and vest or blazer, much to Penny's approval.
Over the course of the series, despite retaining his nerdy habits, Leonard's confidence has grown and he has become more socially adept through out the series.
Leonard is 5' 5" (1.65 m) with brunette hair and brown eyes. His wardrobe includes black thick-framed glasses, neutra-colored sweat jackets, brown trousers, physics-themed T-shirts and low-cut black Converse All Stars sneakers.
All of the members of Leonard Hofstadter's family are accomplished scientists, apart from his younger brother, Michael, who is a very successful lawyer. Leonard is actually the least successful of his family in their respective fields.
His mother, Dr. Beverly Hofstadter, is a psychiatrist and neuroscientist who has a personality identical to Sheldon's, including strict speech patterns, lack of social conventions, and attention to details. She is mainly responsible for the difficult childhood Leonard had. Due to his mother's views, Leonard never celebrated his birthday during his childhood, and when she visited him, she didn't see the point of viewing his unoriginal research since it had already been done.
She never gave him any sort of approval growing up and would humiliate him in public, including forcing him to give back an award at the ceremony because it was too similar to his brother's earlier project. This resulted in his recurrent bed-wetting. In "The Pants Alternative," Leonard suggested the reason that his brother is 8" taller than him is due to his mother's lack of nurturing him as a child. In "The Engagement Reaction," Penny similarly suggests that Leonard's lack of nurturing from his mother "makes him desperate to please women." Leonard had mentioned to Penny that when he was young he built a "hugging machine" with two self-constructed remote-controlled arms in order to compensate for the lack of affection from his mother; the saddest thing was that his father used to borrow it too. Beverly also mentioned to Howard that Leonard's brother and sister are more successful in their respective fields than he is, but she was not proud of them as they weren't "her" accomplishments. When he was six, Leonard had walked in on his naked parents while his mother was spanking his father with his new ping-pong paddle. Leonard had thought that that incident had been a nightmare of his youth.
Leonard's younger brother, Michael, is a Harvard law professor and is married to the youngest appellate court judge in New Jersey history. Leonard also says that his brother is eight inches taller than him.
Leonard's sister is a cutting-edge medical researcher working with adolescent gibbons to cure diabetes. She has even gone as far as successfully growing a human pancreas in one. Leonard also says that she is 38 years old and married in Season 4.
Leonard's father is briefly mentioned as an anthropologist. Sheldon pointed out that Leonard's middle name, "Leakey", comes from famed archaeologist Louis Leakey with whom Leonard's father had worked, which Leonard finds humiliating. It is implied due to Beverly's nature that Leonard prefers his father over her. In Season 2, he had an affair, which resulted in divorce. Leonard's father has not been seen or named. Although his relationship with his father is unknown, it can be presumed that Leonard prefers his father more.
Leonard's now-deceased Uncle Floyd was briefly mentioned in "The Maternal Capacitance" as the only person from his childhood of whom he had fond memories. In "The Codpiece Topology," though, he mentions his Aunt Edna, a very hairy woman, is sweet and would tickle him when they hugged. The closest thing he ever had to a birthday party was when his grandfather died on his birthday, so he got to see his cousins and have his favorite German chocolate cake.
There is male pattern baldness in his family. According to Sheldon, when his uncles sit around the dinner table they look like a half carton of eggs. At Thanksgiving, his grandmother with Alzheimer’s had an episode where she took her clothes off in the apartment; Leonard describing the worst part as when she carved the turkey.
In "The Itchy Brain Simulation," Leonard showed Penny a sweater that an aunt had made for him when he went off to college. It is bright red, very itchy and had "Lenny" spelled out on the front. At the same time as the sweater, his brother got a car from her.
- Father: Dr. Hofstadter
- Mother: Beverly Hofstadter
- Brother: Michael Hofstadter
- Sister: Unnamed Sister
- Uncle: Floyd Hofstadter
- Aunt: Aunt Edna
- Grandmother: Grandmother Hofstadter
Leonard is usually mildly shy around women but, unlike Sheldon, he actually cares about having a girlfriend. He is the most successful of the main four to get into relationships. Although, quite shy around women at first, not really the 'outgoing' type when it came to relationships, he soon got better with women over time. He is actually his most confident when he is in a relationship, examples include:
- being very natural with Alice and indirectly asking her out when he was in a relationship with Priya. ("The Good Guy Fluctuation")
- acting very kind (and oblivious) to Alex, unknown to him that she was flirting with him, while he was in a semi-serious relationship with Penny. ("The Higgs Boson Observation", "The 43 Peculiarity" and "The Egg Salad Equivalency")
As Howard recounts, Leonard's relationships record prior to Stephanie Barnett, excluding the woman with a Ph.D. in French Literature, is approximated by the geometric progression where n = 0, 1, 2; twenty-seven days with Joyce Kim, two booty calls with Leslie Winkle (2.25 days), and a three hour dinner with Penny which would have been two and a half if they ordered the soufflé when they sat down (4.5 hours). His relationship with Stephanie should have ended after twenty minutes (22.5 minutes for n = 3).
- Main article: Joyce Kim
Joyce Kim was one of Leonard's relationships that was widely talked about in his social circle but was never seen until a flashback in "The Staircase Implementation". A North Korean spy attempting to get information on the government project Leonard was working on (a type of Rocket Fuel), the relationship lasted only 27 days before Joyce defected back to North Korea. It is impossible that Leonard had lost his virginity to Joyce Kim due to the fact that Sheldon had interrupted them.
- Main article: Penny
Leonard is instantly captivated by the presence of Penny during the "pilot episode", and from that moment he sets his mission to date her. By the end of the pilot episode, he has already announced to Sheldon "our babies will be smart and beautiful." Sheldon, however, is quick to add, "not to mention imaginary."
His infatuation with Penny becomes the major force that drives the series during the first season. They did go out to dinner one night with Penny thinking that the other guys were going to join them and Leonard making excuses for them. She asks later at her door if this was a date, however Leonard denies it though he doesn't mean it. At the season one finale, Leonard finally managed to ask Penny out, and they had their first date after Penny has a bad breakup.
Subsequently, they broke up because Penny felt she wasn't as smart as Leonard would like. On another occasion, during the visit of Leonard's mother, Leonard and Penny came very close to having sex, but he ruined the moment by talking about their childhood issues with his mother and her father, respectively.
In the second season finale, when the guys were going on a 3-month scientific expedition, Penny finally showed interest in Leonard, but kept her feelings a secret. In the third season premiere, Leonard and Penny finally started a romantic relationship, and made love for the first time. They remain to have a stable relationship and even had the blessing from his mother during her second appearance during episode, "The Maternal Congruence".
The relationship eventually came to an end during "The Wheaton Recurrence" after Penny was not able to tell Leonard that she loved him as well. Leonard and Penny decided during "The Spaghetti Catalyst" that they should stay friends even though they broke up (for Sheldon's sake). Penny's father would later beg Leonard to "not give up on her".
In "The Isolation Permutation", Sheldon became angry when he had to spend the night consoling a heartbroken Amy from being left out of the bridesmaid dress shopping by Penny and Bernadette. He told Howard and Leonard to keep their women in line. Leonard responded that he should not be the person to do that since he was no longer Penny's boyfriend. Sheldon responded that since Leonard was the person who invited Penny to lunch when she moved into the apartment, everything about her is on Leonard.
In "The Ornithophobia Diffusion", Leonard agreed to go out with Penny to a movie just as friends, their first attempt at going out alone together since they broke up. The first-half of the non-date went well, with the two watching a movie that Leonard picked, a documentary about building a dam on a river in South America. Penny actually had to pay for her own ticket after Leonard pointed out that they were not on a date and sex was off the table.
The two later went to Brady's Beer and while Leonard went to get them fries (which Penny had to pay for hers since Leonard did not want her to eat his fries), Penny started flirting with a geeky screenwriter named Kevin. A ditched Leonard started flirting with a woman named Laura, before Penny suddenly interfered. The two ended up saying horrible things to each other, such as Leonard owned two Star Trek costumes that he wore even when it was not Halloween and Penny thought that Cold Wars were fought only during the winter.
They ended the night walking back to the apartment, both looking gloomy. Leonard told that he did not think they were ready to go out as friends and apologized for his behavior. Penny apologized too but said that she liked the take-charge and cocky Leonard. She said goodnight and went to her apartment.
On the spur of the moment in "The Recombination Hypothesis", Leonard asks Penny out on a date after he imagined what getting back with her might be like. Their real date ends successfully and they agreed to take their relationship slow in "The Beta Test Initiation", by treating it as a Beta test and present Bug reports to each other to settle any issues arising. In "The Friendship Contraction", things seem to move along well with their second attempt at a relationship. During a blackout at the apartment, Leonard decided to hang out at Penny's and they had wine, played with a bubble wrap, and attempted to make out before being interrupted by Sheldon.
In "The Werewolf Transformation", Penny learns the game of chess and beats Leonard the first time she plays. They later picked Sheldon up from Amy's apartment after he went there at 3 a.m. with his bongos.
In "The Weekend Vortex", Leonard checks in with Penny whether she would be okay with him spending a whole weekend on a 48-hour Star Wars online gaming marathon. Penny does not seem disturbed by it, which upsets Leonard a little. She tells him that he does not need to check with her for such matter. He, on the other hand thinks that he should, since he does not want to upset her, yet if he does check in with her, he does not want her to think he is being too serious, since they are still at the Beta Test phase of their relationship.
Penny asks him what he expects her to say so Leonard tells her that he expects her to at least show that she is upset at the prospect of him being unavailable for the entire weekend, but realizes that he is a wild stallion that needs to run free. Penny tries it again with the version that Leonard wants and the two share a kiss.
In "The Launch Acceleration", Penny suggested that they move up their relationship and get intimate; however Leonard ruins the mood when he asks her to marry him. They later meet and Penny does have the courage to tell him "no" and not break up with him as she did two years previously when he told her that he loved her. In "The Countdown Reflection", there was some friction between the two of them about the proposal, but when Howard was being launched they were holding hands to comfort each other.
In "The Date Night Variable", Raj who was butting into their date, insisted that Penny tell Leonard how she felt, but she was not ready and threw him out of the apartment. Leonard wants to talk to Penny about their relationship, but Penny says that she has had a hard day and wants to just hang. Penny is expressing doubts about her feelings for Leonard, though she claims to be happy with the relationship in "The Decoupling Fluctuation". She almost tells Leonard about her doubts and then ends up sleeping with him after looking into his sad eyes. Sheldon's pretty new graduate assistant Alex Jensen flirts with Leonard in "The Higgs Boson Observation", which he doesn't respond to, but Penny does take note. She is working up to telling him that she loves him. In "The Holographic Excitation", Penny revs up their relationship by becoming more acquainted with his work and seducing him in his lab.In "The 43 Peculiarity", Penny is working with a fellow English student named Cole, who's working with her on an oral class project. Leonard is jealous and follows him down the apartment stairs and tells him to watch out for "Penny's boyfriend" since he was a gang member. Penny is upset with Leonard and the two later meet in the hallway and have an argument. Penny tells him that she cannot believe that he cannot trust her and that he embarrassed her in front of Cole who knew who he was since his picture is on her refrigerator. Penny continues to tell him that he is the one she is with and accidentally mentions that he knows that she loves him. Leonard is stunned and tells her that that is the first she ever said that. Penny rushes off to work.
In "The Egg Salad Equivalency", after Alex asks Leonard out to dinner and didn't tell Penny about it, she begins to feel really insecure about their relationship. Leonard apologizes by showing up at her door playing a little ditty on his cello. She decided instead of feeling bad, she would do something about it. First, she looks into science classes at college, but they sound boring. Then she buys some geek glasses to look smart, which does have a positive effect on Leonard as he takes her into his bedroom.
In "The Spoiler Alert Segmentation", Sheldon really makes Leonard mad and he says that he wants to live with Penny and not some crazy person. Leonard suggests to Penny that he move in. He asks her if she's all right with it, but she only agrees not to hurt his feelings and even tells Sheldon. She does try to get him admit that Sheldon can't get along without him. Amy plans to move in with Sheldon as roommates. When Amy confronts Penny since she can't move in with Sheldon, Leonard over hears the conversation. Penny tells him that he never asked her if she was ready. Then Amy and Penny go off to drink and talk about their jerky boyfriends and Leonard ends up back in apartment 4A.
In "The Tangible Affection Proof", Leonard takes Penny out to dinner for Valentine's Day. Penny feels that all the efforts never meets the expectations and she will end up disappointed. At dinner, Penny sees an old boyfriend proposing to the girl that broke them up. Leonard again tries to propose and Penny stops him. They begin to argue in front of Howard and Bernadette who have their own issues that night. Later after they barely say good night, Leonard tells Penny that she ruined the whole evening. Penny finally admits that she has commitment issues. She is so happy right now that she knows they are going to get married and since it is forever that scares her. Leonard then puts the responsibility of proposing on her and that she should do it when she's ready. In the end, she does ask him to be her valentine.
In "The Closure Alternative", Penny was concerned about the passion Leonard showed for science, comics and various television shows while she had no passion in her life. Her dreams had always been to become a famous movie star and live a glamorous life. Anything less than that she felt was settling. Her epiphany comes that she is passionate about her boyfriend Leonard and all her wonderful friends.
In "The Bon Voyage Reaction", Leonard takes a job working on a Stephen Hawking expedition in the North Sea for four months over the summer. Penny will miss him, but knows that it is a good opportunity and sends him off. At the airport, she is fussing over him like his wife sending him off on a business trip. She also has to watch Sheldon over the summer, too.
In "The Hofstadter Insufficiency", Leonard hurts Penny because he seems to be having such a great time on the ship, she thinks that he is not even missing her. In "The Deception Verification", Leonard makes up for it by showing up early and hides in her apartment so that they can spend time together a couple before telling the gang that he's back. It works until Amy and Sheldon find them together after they think that Penny is cheating on Leonard. When he was six, Leonard walked in on his naked parents while his mother was spanking his father with his new ping-pong paddle. Leonard had thought that that incident had been a nightmare of his youth. In "The Raiders Minimization", Penny purchases a psychology that Beverly Hofstadter has written about all his problems growing up. Leonard is extremely embarrassment, so Penny does whatever he wants to make him feel better including having sex in his lab. He mentions this to Howard, so Bernadette tells Penny. She dresses up for sex tempting him and then for revenge makes him skpe with his mother to discuss him manipulating his girlfriend.In "The Romance Resonance", Howard makes a romantic gesture to Bernadette by writing her a song. Leonard mentions that Penny never does anything like that, except for sex which doesn't count. Penny is determined to romance Leonard. Raj can only suggest that she cook for him or reenact a scene from a romantic movie. Finally after a candlelight dinner, Penny puts on some romantic music, spreads rose petals into her neatly cleaned bedroom and presents Leonard with a first edition of his favorite book from when he was growing up, The Hitchhiker Guide to the Galaxy. Unfortunately, Leonard had already purchased the book after they had seen it in a used bookstore together. Penny gets upset telling him about all the thoughtful things he had done for her and shows him her collection. An airplane ticket, a rose given to her "just because" and the eleven page letter he wrote her after the first time they slept together. Leonard was very touched that she kept all these things. He then kisses her, showing that Penny did do something romantic.
In "The Thanksgiving Decoupling", Penny reminisces about a Thanksgiving she had in Las Vegas with former boyfriend Zack Johnson which included one of those silly fake Wednesday. Penny appears devastated when her friends inform her that the wedding was real and she is Mrs. Zack Johnson. Penny and Leonard argue about how quickly she should get her annulment so Penny invites Zack over to their Thanksgiving dinner at Howard's mother. Zack appears as dimwitted as ever telling them that breaking up can be hard on the kids which they don't have. Finally, Penny and Leonard make up and she tells him that the next time she gets married it will be for real, for life and (jokingly) or for money.
In "The Cooper Extraction", Amy points out to the gang that most of them would not know each other if Sheldon had not been part of their lives. Penny feels that Leonard would never have successfully asked her out at the Cheesecake Factory, while Leonard suggests that she would have ended up with Zack. In the Zack story, he spent all the rent money on some magic beans. Leonard might have moved in with Raj, however he thinks that he would be eating Raj's cooking too much and would be overweight since he didn't have a girlfriend.In "The Hesitation Ramification" and "The Occupation Recalibration", Penny gets a part in the TV series NCIS and is very disappointed when her scene is cut and not aired. Frustrated and tipsy, she sees her life crumbling when Leonard reminds her that she still has him. She agrees with him and probably to show some advancement in her life, she kneels down and proposes to him. All Leonard can utter is "Umm", much like when he wouldn't succumb to her drunk advances after her Halloween party in "The Middle Earth Paradigm". Penny gets very upset since she doesn't think he wants to marry her. After she storms out, Leonard is afraid to ask her if their relationship is now over. The next day/episode he does visit Penny who assures him that they are still together and that he did the right thing not agreeing to her proposal because she was a mess. Now she has rethought her carreer and decides to quit the Cheescake Factory and focus all of her time on acting. Leonard does support her though he is obiviously worried about her. The proposal count currently is two to one.
In "The Table Polarization", the gang wants to add a dining room table to Apartment 4A which Sheldon objects to. Penny gets Leonard to pick out a dining set that she likes and gets Leonard to stand up more against Sheldon. Sheldon objects to Penny changing Leonard to stand up against him. Penny points out that Amy has changed Sheldon who agrees and then decides to break up with Amy.
In "The Mommy Observation", Penny seems to feel that if she were a successful actress and had to move around, Leonard would leave his job even if it was a tenured positon to follow her since she would be making more money.
- Main article: Elizabeth Plimpton
Dr.Elizabeth Plimpton is a colleague of Sheldon who stays with Leonard and Sheldon when she visits Caltech for a conference. The first night, she seduces Leonard before attempting to set up a gang-bang with Leonard, Howard, and Raj. Raj gets rid of the other two. Leonard did not pursue a relationship past the one-night stand.
- Main article: Leslie Winkle
Leslie Winkle is a fellow scientist who works in the same lab as Leonard. The first time Leonard asked Leslie out, he was rejected. His further relationships with her proved to be only short-lived casual sexual intercourse. Prior to his later encounters with Wil Wheaton, Sheldon considered Leslie his archenemy and one of their disputes resulted in the final breakup between Leslie and Leonard after Leonard supported Sheldon's theories over her own. After learning from a drunken Penny that friends can have meaningless sex, in the season finale of Season 3 he approaches Leslie who turns him down and has not again been seen on the series.
Interestingly, Leonard's and Leslie's actors also had a relationship in the older sitcom, Roseanne. In this sitcom, their tumultuous relationship resulted in marriage and a baby girl by the time the series ended.
Dr. Stephanie Barnett
- Main article: Dr. Stephanie Barnett
Leonard was indirectly introduced to Dr. Stephanie Barnett by Howard, who was interested in her. Leonard then stole her from Howard and started a stable relationship with her. Although the relationship went well, even earning Sheldon's approval, Leonard felt uncomfortable with Stephanie moving in with him and the speed of their relationship, and they broke up, the details of their breakup—why and when—unknown.
- Main article: Mrs. Latham
Mrs Latham appears in "The Benefactor Factor" and is played by actress Jessica Walter. The gang is attending a Saturday night fund raiser for Cal Tech and they meet Mrs. Latham, a major university donor. She likes to throw zingers at people and make them uncomfortable, which she considers an advantage to being rich. Mrs. Latham invites Leonard to dinner to talk more about his research. After the dinner, she was flirting with Leonard on the way home. She kisses him on the lips, causing great discomfort towards Leonard. The next night, Mrs. Latham asks Leonard if he's hungry. He replies with "For food, right?", indicating indirectly that he is uncomfortable. She picks this up right away. However, she tells Leonard that she was going to donate the money to Cal Tech whether anything happened or not and hints to Leonard that she is skilled enough in the art of love-making to land her rich husband. Once Leonard realizes that he is not just being used, he gives in. The following morning, Leonard gets home just as Penny is going out for a jog. She kids him about doing the "walk of shame".
- Main article: Priya Koothrappali
In "The Cohabitation Formulation", Priya and Leonard discussed their relationship and decided to start dating each other, much to Raj's dismay. In "The Roommate Transmogrification", it was revealed that Priya was moving back to India. In spite of the physical distance between them, Priya and Leonard are at present attempting to maintain a long distance relationship, communicating via Skype.
Leonard and Priya's relationship seems to have come to an end after Leonard admitted to making out with a comic book artist and admitting it to Priya and apologizing, Priya then admitted she slept with her ex-boyfriend. Leonard then closed the laptop and he claimed he was single in "The Isolation Permutation" after leaving Sheldon at Amy's apartment.
- Main article: Alice
Alice is a girl who meets Leonard at the comic book store and then seduces him. This occurs during the period when Leonard is having a long distance relationship with Priya. Leonard initially gives in to temptation, but at the last minute pulls away. Next time he's communicating, via the webcam, with Priya, he confesses his weakness. To his surprise, Priya has no problems with it. Upon questioning, Priya reveals that she has not been faithful to Leonard resulting in them breaking up.
AlexAlex Jensen is a Caltech physics doctoral student and Sheldon's new research assistant. Alex first triggers Amy's jealousy; however, when she and Penny see her flirting with Leonard, Amy says that Alex is Penny's problem. When Penny introduces herself, Alex mistakes Penny for Leonard's assistant. Penny says that they were working on a five year project. Alex replies that she is sure that a lot of people would want to work with someone as talented as Leonard. Penny quips that a lot of people can't and then she takes Leonard to her apartment for a romantic tryst. As Penny is leaving with Leonard, Alex remarks that she seems nice sounding like she has no ulterior motives outside of working with Sheldon. Alex is next seen in "The 43 Peculiarity" talking to Raj and Howard. In the Caltech cafeteria she spots Leonard who tells her to call him "Leonard" and not Dr. Hofstadter. He discusses his jealousy of Penny's English classmate whom he thinks is hitting on her. Alex says that its probably nothing and that he probably gets hit on all the time. Leonard laughs it off, though she tells him that he probably gets hit on without even knowing it. Leonard doesn't pick up on her signals that she really likes him. He leaves as she looks a bit depressed. Later she texts him that they can get together and talk any time he wants. Leonard thinks that she is just being friendly. In "The Egg Salad Equivalency", Alex asks Leonard out to dinner after he expresses an interest in the Physics lecture she is going to and this really startles Leonard. Though it makes him feel good "to the bones" to have an attractive young woman interested in him, he tells everyone he still loves Penny and wouldn't do anything about it. He mentions Alex's actions to Sheldon who tries to talk to her about it and lands the whole gang in the Human Relations Administrator's office. He also didn't tell Penny about and Alex's actions make her feel really insecure about their relationship. Leonard apologizes by showing up at her door playing a little ditty on his cello.
Other mentioned girlfriends
- French woman who had a PhD in French literature.
- Karen Berberick - High school girlfriend where Leonard took their relationship so slow, she didn't even know they were dating.
- Leonard wants to achieve a Nobel Prize someday.
- Leonard is skeptical of loop quantum gravity.
- While Leonard's IQ is never directly stated in the show, he states in the Pilot that he and Sheldon have a combined IQ of 360. Sheldon states that his IQ is 187, so therefore Leonard must have an IQ of 173. However, Sheldon also states his intellect cannot be accurately measured by a "regular" IQ test.
- Leonard received his PhD at age 24.
- Leonard is 5 ft 5 in (1.65 m) tall  – the height of Johnny Galecki. In "The Egg Salad Equivalency", Sheldon states his "short, bespectacled colleague" is "... strutting around like he’s five-foot-six," which implies that Leonard is shorter than this measurement. (Refer to the Trivia of "The Decoupling Fluctuation".)
- In "The Alien Parasite Hypothesis", which aired in 2010, Leonard says, "12 years after high school, and I’m still at the nerd table." This implies he was born in 1980, the same year as Sheldon, provided the following. The assumption that he graduated at the normal age of 18 years old is supported by him having had a senior prom, which is mentioned in "The Peanut Reaction".
- Leonard has a grandmother with Alzheimer's that came to Thanksgiving and stripped naked, carving the turkey.
- Leonard wears boxer shorts rather than underpants or trunks.
- Leonard always wears neutral colored sweat jackets.
- Leonard wears black framed glasses (he originally wore tortoise shell glasses eight years ago), even if in costume.
- Despite his lactose-intolerance, however, Leonard can actually process small amounts of non-fat ice-cream without farting (according to Sheldon).
- Leonard has Darth Vader 'No More Tears' shampoo and Luke Skywalker hair conditioner.
- Leonard is a huge Superman fan, so much so that he owns all of the Superman films and more than 2600 Superman comics, as well as Superman underwear. He also has an unseen Superman costume with new boots that adds three inches to his height, which he wore at the comic book store's annual New Year's Eve party when the group went as the Justice League. However, Sheldon says he is their weak point, and he got replaced by Penny's friend Zack at the most recent party.
- Sheldon reveals that for everything his password is 'Kal El'.
- Many of Leonard's shirts are scientifically inspired, such as in episode 1x03 where he can be seen wearing a shirt with a "Paradox" molecule.
- Leonard and Sheldon own Dell XPS computers. After "The Bozeman Reaction," he owned a Dell Adamo XPS.
- Leonard can play the cello, mirroring Johnny Galecki's, the actor who plays Leonard, own talents.
- Leonard and Sheldon are named after actor and producer Sheldon Leonard.
- Leonard is unaccustomed to observing birthdays, by choice of his mother's views on psychiatry. He had never had a birthday party before moving to Pasadena.
- Leonard's parents live in New Jersey.
- Leonard drove a silver Volvo with a luggage rack on top early on in the show.
- Leonard plays lead guitar on Rock Band.
- Leonard plays Age of Conan with the guys and is probably an Assassin (his character can be seen when he is checking on Penny via the game).
- Leonard has sleep apnea.
- Leonard gets carsick unless he sits in the front.
- Leonard has trouble making eye contact, only being able to do so with Penny and the rest of his friends, which could be indicative of Aspergers Syndrome, ADD, or simply social anxiety.
- Leonard is the group's (not including Penny) Wii bowling 'Team Captain'.
- Leonard's last name (Hofstadter) is in honor of Nobel Prize Winning Physicist Robert Hofstadter, who worked at Stanford University in the field of Nuclear Physics.
- Leonard dislikes his name since it has "Nerd" in it, saying he prefers the name Angelo as it has "angel" and "jello" in it, although he was high at the time.
- Leonard tried wearing contacts in 7th grade, but could never get used to them.
- Leonard claimed he could hear Penny flushing her toilet from his apartment.
- Leonard was his school's debate club president.
- Leonard used to get stuffed in his own cello case.
- Leonard believes he is the "King of foreplay", as he used to do lengthy shows for Penny before they had sex.
- Leonard gets asthma attacks when he has panic attacks.
- He is only main male character who wears glasses.
- He is the only character who has had five girlfriends Joyce Kim, Leslie Winkle, Penny, Stephanie Barnett and Priya Koothrappali), followed by Howard with four girlfriends (Christy Vanderbel, Leslie Winkle, Lisa and Bernadette Rostenkowski).
- He is afraid of horses.
- Despite being short, Leonard states he is "too tall to be a jockey" (Though to Sheldon's amusement he's the perfect person for the sport).
- Leonard always wears shorts when he goes to Kurt's apartment for Penny's errands.
- Leonard is a quarter inch taller than Howard.
- Leonard is always wearing a t-shirt when he is in bed with Penny.
- Like Sheldon, Penny, Rajesh, and Amy, Leonard became a reverend to marry Howard and Bernadette.
- Leonard's great-great grandmother was half-Cherokee (as stated in "The Irish Pub Formulation"). This makes Leonard 1/32nd Cherokee.
- Estimated by Howard; Leonard's ideal mate is a short, low self-esteemed woman scientist living in a government facility in China.
- As he stated in "The Contractual Obligation Implementation," when he was in middle school he wanted to be a rap star like Snoop Dog, but with "a healthy respect for the police." However, he states his parents pushed him into becoming a scientist.
- One of Leonard's favorite fictional characters is Frodo Baggins from the Lord of the Rings (as stated by Sheldon in "The Large Hadron Collision".)
- Leonard finds himself dating a married woman in "The Thanksgiving Decoupling".