Leslie Wolff told The Associated Press that the 67-year-old was diagnosed four or five weeks ago and is under hospice care. Wolff said that Frazier's loved ones are hoping for the best but called it a "serious condition."
Frazier was the first fighter to beat Muhammad Ali, knocking him down and taking a decision in what was called the Fight of the Century in 1971. He would go on to lose two more fights to Ali, including the epic "Thrilla in Manilla" 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.
The Associated Press contributed to this report.