The 67-year-old boxer was diagnosed four or five weeks ago, Frazier's personal and business manager said Saturday. Leslie Wolff told The Associated Press that doctors have not yet told Frazier how long he has to live.
"We have medical experts looking into the all the options that are out there," Wolff said. "There are very few. But that doesn't mean we're going to stop looking."
Wolff, who has been Frazier's manager for seven years, said the boxer had been in out and out of the hospital since early October and receiving hospice treatment the last week.
"We appreciate every prayer we can get," Wolff said. "I've got everybody praying for him. We"ll just keep our fingers crossed and hope for a miracle."
Frazier's illness was first reported by the New York Post, citing an unidentified source.
Frazier was the first man to beat Muhammad Ali, knocking him down and taking a decision in the so-called Fight of the Century in 1971. He would go on to lose two more fights to Ali, including the epic "Thrilla in Manila" bout.
Frazier was bitter for many years about the way Ali treated him then. More recently, he said he had forgiven Ali for repeatedly taunting him.
Smokin' Joe was a small yet ferocious fighter who smothered his opponents with punches, including a devastating left hook he used to end many of his fights early. It was the left hook that dropped Ali in the 15th round of their "Fight of the Century" at Madison Square Garden in 1971 to seal a win in a bout where each fighter earned an unheard of $2.5 million.
While that fight is celebrated in boxing lore, Ali and Frazier put on an even better show in their third fight, held in a sweltering arena in Manila as part of Ali's world tour of fights in 1975. Nearly blinded by Ali's punches, Frazier still wanted to go out for the 15th round of the fight but was held back by trainer Eddie Futch in a bout Ali would later say was the closest thing to death he could imagine.
Frazier won the heavyweight title in 1970 by stopping Jimmy Ellis in the fifth round of their fight at Madison Square Garden. Frazier defended it successfully four times before George Foreman knocked him down six times in the first two rounds to take the title from him in 1973.
Frazier would never be heavyweight champion again.
In recent years, Frazier had been doing regular autograph appearances, including one in Las Vegas the weekend of a Floyd Mayweather Jr. fight in September.
"I was very sad to hear the news. It's a tragedy," leading British promoter Frank Warren said. "He's one of the greatest fighters of his generation and one of the best heavyweights in history. It's a sad thing and I know everyone in boxing will be wishing him well."