Neil Simon, 91, the prolific playwright whose string of hit comedies won the acclaim of delighted audiences and a less enthusiastic reception from critics, died Sunday.

According to Bill Evans, Mr. Simon's longtime friend and the Shubert Organization director of media relations, the playwright died early Sunday of complications from pneumonia in a Manhattan hospital, the Associated Press reported.

Mr. Simon had suffered from kidney problems for several years and was on dialysis three times a week. In 2004, Evans donated a kidney, and Mr. Simon underwent a successful transplant.

At the height of his powers and popularity, Mr. Simon was a one-man assembly line who turned out comedies built on accessible relationships and situations and crammed with polished one-liners. His detractors sniped that his work was middlebrow and unchallenging, and joked that he would write a play whether he had an idea for one or not.

But Mr. Simon could afford to ignore them. The guffaws in the theater for The Odd Couple and many other smashes allowed him to laugh all the way to the bank.

Jack Lemmon, who starred in several Simon-inspired films, once summed up his gift by noting: "Neil had the ability to write characters — even the leading characters that we are supposed to root for — that are absolutely flawed. They have foibles. They have faults. But they are human beings. They are not all bad or all good. They are people we know."

Mr. Simon was by far the most commercially successful and performed playwright of the last 60 years. He once had the satisfaction of seeing no less than four of his shows running simultaneously on Broadway. It happened in the 1966-67 season with Barefoot in the Park, The Odd Couple, the musical Sweet Charity, (he wrote the book) and Star-Spangled Girl.

Although his humor was quintessentially American, it traveled well and enjoyed wide appeal. His plays were staged in dozens of languages in productions around the world.

In his prime decades in the 1960s and '70s, Mr. Simon's reach extended far beyond the limits of the theatrical audience because so many of his Broadway shows went straight to Hollywood to become films that did very well at the box office.

Because he chose to leave audiences laughing for most of his career, Mr. Simon fell victim to the perennially unfair bias against comedy and comedians in the traditional pecking order of artistic esteem.

As the critic Walter Kerr once observed of him, "Whenever a playwright manages to be hilariously funny all night long … he is in immediate danger of being condescended to." And no one would deny that Mr. Simon, who wrote 33 plays, from Come Blow Your Horn in 1961 to Rose's Dilemma in 2004, could be very funny, with the zingers springing naturally from the predicaments of his characters.

In The Odd Couple, the slovenly sportswriter Oscar Madison erupts at the terminally neurotic ways of his roommate, Felix Unger, and bellows: "You leave me little notes on my pillow! I told you 158 times I can't stand little notes on my pillow! 'We're all out of cornflakes. F.U.' Took me three hours to figure out F.U. was Felix Unger!"

It was only in the later stages of his career, when Mr. Simon struck a more serious and reflective note in directly autobiographical plays, that he began to gain the critical recognition and honors that his admirers thought were long overdue.

In a literal sense, Mr. Simon put himself into his work, and his own experiences and relationships became fodder for his comedies and then for his more pensive dramas. As he himself wrote in The Play Goes On, "I always knew you could trace my life through my plays." He added that the recurring motif in his body of work was the theme of abandonment, from the betrayed mother in Broadway Bound to the widower in Chapter Two.

Marvin Neil Simon was born in the Bronx on Independence Day 1927. He grew up in a working-class Jewish household that would later furnish material for some of his finest work. He attended New York University and then joined the Army in 1946, where he began his writing career for the camp newspaper.

When he left the service, Mr. Simon returned to New York and teamed with his brother Danny to write comedy for radio. Eventually, he moved into television and joined the legendary laugh factory that produced scripts for Sid Caesar's Your Show of Shows and Phil Silvers. The team included Mel Brooks, Woody Allen, Larry Gelbart and Carl Reiner.

In his memoir Rewrites, Mr. Simon said it was his years in the riotous company of these kings of comedy that shaped him most as a writer. He saluted that cherished halcyon era in television with his play Laughter on the 23d Floor.

Mr. Simon started mining his experience early in his first big Broadway hits, Come Blow Your Horn and Barefoot in the Park – which launched the career of a then-little-known Robert Redford. The following years saw a steady and lucrative stream: Plaza SuiteLast of the Red Hot LoversPromises, Promises and The Sunshine Boys.

"My first play [Come Blow Your Horn] was fictionalized autobiography," Mr. Simon recalled in an interview with the Inquirer and Daily News. "But it was done as a farce because that's all I knew how to write. I never felt a sense of betrayal, that I was exposing the frailties of the people in my family, including myself. I didn't dig deeply enough for that. I got closer to it when I wrote Chapter Two."

Chapter Two, which opened on Broadway in 1977 and was released as a movie in 1979, addresses marriage and grief in an especially personal and painful way. Many critics have chosen it as his finest work. Mr. Simon lost his first wife, Joan, to cancer in 1973 at the age of 39. They were married 19 years and had two daughters, Ellen and Nancy. In all, Mr. Simon was married five times, twice to the same woman, according to the Associated Press.

Chapter Two deals with the loss of a first wife, and the courting of a second. In the film, the new woman was played by Marsha Mason, who married Mr. Simon. Mr. Simon subsequently married Diane Lander in 1987 and divorced her two years later. They remarried in 1990.

After Chapter Two, Mr. Simon looked back to his coming of age and produced a trilogy – Brighton Beach Memoirs, Biloxi Blues, and Broadway Bound – that stands far above his previous comedies. "I care more about them [the three plays], not only because they are more personal, but because it's a new style," he said. "It goes a lot deeper than I've been before, even in Chapter Two."

The three interlocking plays focus on sibling rivalries and struggles among parents and children. The story culminates in Broadway Bound with the efforts of Eugene Jerome to cope with the breakup of his parents' marriage. Throughout the three works, the central issue is the pain family members can inflict on one another, often in the name of love.

These plays gave Mr. Simon a new stature, and in 1992, he received the Pulitzer Prize for Lost in Yonkers.

Three of Mr. Simon's works – The Odd Couple, Biloxi Blues, and Lost in Yonkers – won the Tony Award for best play. Besides the plays that became films, Mr. Simon wrote a dozen original screenplays. He was nominated for an Oscar four times – for The Odd Couple (1968), The Sunshine Boys (1975), The Goodbye Girl (1977), and California Suite (1978).

Act II Playhouse in Ambler is in rehearsals for Mr. Simon's Biloxi Blues, scheduled to run Aug. 28-Sept. 23. Tony Braithwaite, artistic director for Act II and director of the show, said news of Mr. Simon's passing came "literally while we were rehearsing." Since Braithwaite was coincidentally traveling to Manhattan on Sunday, the cast signed a playbill he planned to leave at the spontaneous memorial at the door of the Neil Simon Theatre in New York.

Mr. Simon's plays have often appeared at Act II. "He's especially good with the foibles of the bourgeoisie," Braithwaite said. "Our audience gets it. He speaks their language. His characters are deeply relatable, deeply human." That relatability, he said, has made the playwright popular well beyond his native shores: "Anyone who has been in a relationship can relate to the struggles of Oscar and Felix in The Odd Couple," he said. "They call Biloxi Blues a 'World War II play,' but it's much more than that: It also explores racism, homophobia, falling in love, the nature of authority, and being on your own. He really wrote for everyone."

After Lost in Yonkers, Mr. Simon's audience dwindled markedly in both theaters and multiplexes. In 1991, he got in a much-publicized spat with Kim Basinger and Alec Baldwin, the stars of The Marrying Man, and vowed he would never do another screenplay. He relented to write the poorly received Odd Couple II.

Mr. Simon's last two Broadway shows – 45 Seconds to Broadway and Rose's Dilemma – also flopped. Another feud with the latter's star, Mary Tyler Moore, led to her angry departure and more bad publicity.

Ultimately, Mr. Simon could look back on the compass of a career that was rich in more than profits. He once reflected, "I made my reputation with comedy, and I was quite happy to entertain. Good comedy is hard to come by. But when you want to do more personal, deeply felt stories, you can't do them through comedy. You've got to get in the trenches and dig."

TMZ reported Sunday that a memorial to Mr. Simon would be held in New York on Thursday.

Staff writer John Timpane contributed to this article.