Morrison's longtime promoter, Tony Holden, said Morrison died at 11:50 p.m. with his wife, Trisha, beside him.
Morrison tested positive for HIV in 1996 before a fight with Arthur Weathers, effectively ending his boxing career. In the years that followed, he denied having HIV and also challenged the existence of the virus.
Trisha Morrison, who married Morrison in 2011, picked up that fight, and in a recent interview with ESPN.com insisted that Morrison had Guillain-Barre Syndrome, not HIV.
Holden declined to comment Monday on the cause of Morrison's death.
"I don't know what the official cause of death at the hospital will be," he said.
"You prepare for things like this, and still you feel like you got hit by a truck when you hear the news."
In 1993, Morrison beat George Foreman to win the WBO heavyweight title, only to lose it to unheralded Michael Bentt in a defeat that scuttled a showdown with Lennox Lewis. Morrison would fight Lewis in 1995, getting knocked out in the sixth round in Atlantic City, N.J.
Morrison won his first 28 professional fights, beating faded champions such as Pinklon Thomas along the way. His career reached its apex in the summer of 1993 with the unanimous decision over Foreman, then in the midst of a comeback, to claim a vacant title.
Morrison's license was quickly suspended in 1996 by Nevada after his positive HIV test, and the ban was, in effect, upheld by every other sanctioning body. Morrison said at a news conference he'd never fight again, blaming his plight on a "permissive, fast and reckless lifestyle.''
His lifestyle never changed, though, even when he stepped away from the ring.
He had already run afoul of the law in 1993, when he pleaded guilty to assaulting a college student. He also dealt with weapons charges and multiple DUI incidents over the years. He was sentenced to two years in prison in 2000, and another year was added to his sentence in 2002 for violating parole.
When he was released, Morrison said his HIV tests had resulted in false positives, and he wanted to resume his career. He passed medical tests in Arizona -- even as Nevada stood by its decision -- and returned to the ring. Morrison fought twice more in his career, winning once in West Virginia and for the final time in Mexico. He finished with a record of 48-3-1 with 42 knockouts.
He also gained fame for his role in the 1990 movie "Rocky V," in which he portrayed Tommy Gunn, a rookie boxer who is trained by Rocky Balboa (Sylvester Stallone). Gunn goes on to win the heavyweight title and then later fights and loses to his mentor.
Morrison was born in Arkansas and grew up in Oklahoma.
