Palillo suffered an apparent heart attack at his home about 4 a.m., said Karen Poindexter, a close friend of the actor. He was pronounced dead at Palm Beach Gardens Medical Center.
Palillo was inextricably linked with the character he played from 1975 to 1979 on "Kotter," the ABC sitcom, in which the title character returns to his Brooklyn alma mater to teach a group of loveable wiseguys known as the Sweathogs. Horshack was the nasally teen who yelped, "Oooh, ooh," and shot his hand skyward whenever Kotter posed a question.
The show was a ratings success and pop cultural phenomenon, injecting smart-Alec phrases such as "Up your nose with a rubber hose" into the mainstream and propelling co-star John Travolta to stardom. But the series only lasted as long as a high school education and its end, for Palillo, brought difficulty.
He said he felt exiled throughout the 1980s, unable to find parts, sinking into depression, and rarely venturing from his apartment. When offers did come, he felt typecast as Horshack.
"While I loved him, I really loved him, I didn't want to do him forever," he told the Birmingham News in 1994.
Ronald Paolillo was born April 2, 1949, in Cheshire, Conn., eventually dropping the first "o'' from his surname. His father died of lung cancer when he was 10 and he developed a stutter. His mother thought getting him involved in a local theater might help. He fell in love with the stage and overcame his speech impediment.
He attended the University of Connecticut and earned parts in Shakespearean productions before his big break.
When he auditioned for "Kotter," he thought he'd be passed over for others who had more of a tough-guy New York look. He told interviewers that his dying father's voice inspired his character's trademark wheezing laugh. And he said Horshack tapped into feelings any teen could relate to.
"I think he was the smartest kid in school," he told the Miami Herald in 2009. "He was giving up his aptitude in order to be liked. Then and now, that is a very common thing in teenagers."
Palillo went on to get a host of bit parts in shows from "The Love Boat" to "Cagney and Lacey" to "The A-Team," and played himself for a time on the series "Ellen." But "Kotter" remained his most well-known acting part, and he focused on stage directing and writing.
His last act in life mirrored his most famous one, in a real-life classroom instead of one at the fictional James Buchanan High School. Palillo taught acting at G-Star School of the Arts, a high school in West Palm Beach. He was due to return for the school year Tuesday morning, Poindexter said, and classes were to resume next week.
Palillo is survived by his partner of 41 years, Joseph Gramm; two brothers, and a sister. Poindexter said that while her friend might, at times, have resented the shadow Horshack cast over him, he remained fond of the character and knew the part was always more of a blessing than a curse. He remained close to his co-stars, she said, and knew how closely fans related to the characters.
"All of us have been or known one of those Sweathogs," he told The Los Angeles Times last year.