Reggie Lewis, the Boston Celtics' star who collapsed with a heart ailment during a playoff game last April and was warned by doctors that he would risk his life if he ever played basketball again, died last night after collapsing while shooting baskets at the team's training center at Brandeis University in Waltham, Mass.

He was 27 and had resumed light workouts after seeking a second medical opinion and being told he really didn't have a severe heart ailment at all.

After he collapsed on the Brandeis court shortlty after 5 P.M. yesterday with what doctors later described as full cardiac arrest, bystanders, paramedics and Brandeis police officers tried in vain to revive him. He was taken at 5:41 P.M. to Waltham-Weston Hospital, where doctors treated him for another two hours before pronouncing him dead at 7:30 P.M.

Although announcements of his death were soon being broadcast over Boston radio and television stations and nationally televised baseball games, at the request of Lewis's wife, Donna Harris, hospital officals refused to give out any information on Lewis until almost three hours later.

By then the hospital had become a magnet for members of the Celtic family, including Lewis's teammate Rick Fox and two former stars, Dave Cowens and M. L. Carr. "He was a great athlete," Cowens said. "He was one of the guys who bucked the odds and became a professional athlete."

Describing Lewis as "a gentle, kind, wonderful, considerate guy," Dave Gavitt, the Celtics' senior executive vice president, said: "We've lost a very treasured member of our family. It's a time of incredible grief."

Lewis, according to witnesses, spent the final hours of his conscious life doing what he had been doing every chance he could get since he was a schoolboy growing up in Baltimore: shooting baskets with a friend.

After arriving at the gym about 4 P.M., he spent the next hour or so shooting baskets without any strenuous exertion and chatting with onlookers, according to witnesses, including participants and counselors at a women's summer basketball camp.

At 5 P.M., Joseph McDonald, the Brandeis director of public safety, glanced at his monitor of a security camera in the gym and saw Lewis talking to a group of the female players.

Fifteen minutes later, McDonald recalled later, Lewis sank to the floor in a sitting position and soon drew the attention of his friend and a woman who had been shooting baskets nearby and noticed that he was having trouble breathing. The woman, a Brandeis student who asked not to be identified, is a trained medical technician, according to McDonald, who said she was the first to find that Lewis had no pulse and was not breathing and the first to try to revive him.

Once his breathing stopped, he never regained consciousness. A Debate About Playing

The death of the man who had been captain of the Celtics recalled at once the 1986 cocaine-related death of Len Bias, which occurred less than 48 hours after the Celtics had made him their top draft choice and second pick over all, and the 1991 death of Hank Gathers, the Loyola Marymount player, who succumbed to cardiomyopathy, a disease that causes the heart to beat less vigorously. That was the same heart ailment that some doctors said had led to Lewis's collapse last April.

It was an opinion that Lewis, who was hospitalized for 11 days after he collapsed during a playoff game against the Charlotte Hornets on April 29, didn't want to hear. And after it was delivered by a team of 11 cardiologists who had examined records of medical tests done while Lewis was at New England Baptist Hospital, Lewis abruptly switched hospitals, moving to Brigham and Women's Hospital in Boston. After a week of further tests, Dr. Gilbert Mudge gave a second opinion, that Lewis did not have a life-threarening heart ailment but merely suffered from a comparatively benign neural condition, neurocardiogenic syncope, in which confusing signals regulate the speed of the heart beat. Dr. Mudge said the condition could be successfully treated with drugs, and that Lewis could safely resume his career.

The conflicting opinions caused a running controversy in Boston, especially after one of the original cardiologists expressed doubt about the new diagnosis, and Lewis, himself, was critical of the Celtic team physician, Dr. Arnold Scheller, for releasing a statement saying Lewis he had potentially life-threatening "cardiac abnormalities."

Confused by the conflicting medical opinions following his collapse in April, Lewis may have sought even further medical advice. He apparently felt confident enough to shoot baskets at the Brandeis training center even though the Celtics did not allow him to take part in team workouts. Team-Leading Scorer

Until his collapse in April, Lewis, a native of Baltimore who played for the city's famous Dunbar High School and later starred for Northeastern University in Boston, had been fixture on the Celtics since the team made him its first-round choice in the 1987 draft, taking him 22d over all.

During his Dunbar years, Lewis played with such later National Basketball Association stars as Tyrone Bogues, David Wingate and Reggie Williams on a team that went 50-0 during one season.

As a four-year starter at Northeastern, he became the Huskies' leading career scorer, with a total of 2,708 points.

Playing both guard and forward for the Celtics, the 6-7 Lewis emerged as both a high scorer and a team leader. He led the team in scoring, averaging 20.8 points a game last season and was selected as the team's captain.

R Lewis