Ritter became ill Thursday while working on ABC's "8 Simple Rules ... For Dating My Teenage Daughter," said Susan Wilcox, his assistant of 22 years. The show became Ritter's TV comeback vehicle.

He died at Providence St. Joseph Medical Center shortly after 10 p.m. Thursday, six days before his 55th birthday. The cause of death was a tear in the aorta, the result of an unrecognized flaw in his heart, said his publicist, Lisa Kasteler.

"It's just stunning, unbelievable," Wilcox said. "Everybody loved John Ritter. Everybody loved working with him. ... Whatever set he was working on, he made it a very fun place."

"It's like there is a big tear in the world's heart," actor Henry Winkler told "Entertainment Tonight." Winkler, best known for "Happy Days," was to make a guest appearance on "8 Simple Rules" and was on the set Thursday. He said he got a call later about Ritter's death.

"He was extraordinary in every aspect of his life, especially as a father," Winkler said. "His children were there at every moment of his life."

Ritter, the youngest son of Western film star and country musician Tex Ritter and actress Dorothy Fay, came to prominence for his role in "Three's Company" and had appeared in more than 25 television movies, a number of films and on Broadway.

He made his successful return to sitcom acting last year with "8 Simple Rules." The show was scheduled to begin its second season Sept. 23.

At the Burbank hospital where he died, Ritter was accompanied by producers and co-workers, his wife, Amy Yasbeck, and 23-year-old son Jason, Wilcox said. He is also survived by three other children.

ABC released a statement saying: "All of us at ABC, Touchstone Television and The Walt Disney Company are shocked and heartbroken at the terrible news of John's passing. Our thoughts and prayers are with his wife and children at this very difficult time."

Ritter graduated from Hollywood High School and earned a degree in drama from the University of Southern California.

"I was the class clown, but I was also student body president in high school," he told The Associated Press in a 1992 interview. "I had my serious side - I idolized Bobby Kennedy, he was my role model. But so was Jerry Lewis."

His first steady job was his role as a minister in television's "The Waltons" in the early 1970s.

With "Three's Company," starting in 1977, his career took off. His other performances included 1996's Oscar-winning movie "Sling Blade" and a Broadway run in Neil Simon's "The Dinner Party." He received an Emmy and other awards for "Three's Company" and was honored by the Los Angeles Music Center in June with a lifetime achievement award.

"Three's Company," about a bachelor sharing an apartment with two attractive women, Suzanne Somers and Joyce DeWitt, was considered racy during its run from 1977 to 1984. And Ritter worried about falling into a typecasting trap after the show ended.

"I would get scripts about 'a young swinging bachelor on the make,' and I said 'No, I've done that,"' he told the AP in the 1992 interview. "Or they'd say, 'You're living alone and ...'

"What I was looking for in my time off was something a little bit different, a little serious, or funny in a different way."

Ritter described his time on the show as "an education" in quick-study acting.

"When the curtain went up, no matter how long you've studied or haven't studied at all, you had to answer to the audience. We didn't do retakes. If there was a (microphone) boom in the shot, so be it," he said.

With "Three's Company," Ritter took material that was considered risque at the time and made it acceptable because of his approachable, self-effacing personality, said Tim Brooks, author of "The Complete Guide to Prime-Time Network and Cable TV Shows."

"He never aspired to be Hamlet," Brooks said. "He was a true actor of the people and television viewers really bonded with him as a result."

Ritter later starred in the television series "Hooperman" and the early 1990s political comedy "Hearts Afire." He received Emmy nominations for his PBS role as the voice of "Clifford the Big Red Dog" on the animated series.

His TV movie appearances included "Unnatural Causes," Stephen King's "It" and "Chance of a Lifetime."

Ritter won popularity among independent film directors in recent years and appeared in films including "Tadpole" in 2002, and the new feature "Manhood." He appears alongside Billy Bob Thornton in the scheduled November release from Miramax "Bad Santa."

Ritter was married from 1977 to 1996 to Nancy Morgan, the mother of his three oldest children, Jason, Carly and Tyler. He married actress Yasbeck in 1999, the mother of Stella.