The disease of "Smokin 'Joe" was hospitalized in Philadelphia, was diagnosed about a month ago, said Leslie Wolff told Reuters.
"We're doing everything we can, but I'd be lying if I did not tell you it's very serious," he said.
Frazier, famous for his three legendary fights against Muhammad Ali in the 1970s, is 67 years old.
Leslie Wolff said Joe Frazier, Olympic champion in Tokyo in 1964 and world champion from 1970 to 1973, was in a "very painful and very serious."
"Joe is a fighter, Joe does not give up," he nevertheless wished to add.
In the annals of boxing, Frazier remains bound for eternity to his great rival, Muhammad Ali, whom he defeated in the "fight of the century" at Madison Square Garden in New York March 8, 1971, before losing the following two rounds.
Joe Frazier, who bowed as against Muhammad Ali and George Foreman in his career, had retired in 1976 before a brief return to the ring in 1981.