(born Jan. 17, 1942, Louisville, Ken., U.S.) U.S. boxer. Cassius Clay took up boxing at the age of 12 and rose through the amateur ranks to win the Olympic light heavyweight crown in 1960. His first professional heavyweight title win was against Sonny Liston in 1964. After defending the title nine times between 1965 and 1967, he was stripped of it for refusing induction into the armed forces following his acceptance of the teachings of the Nation of Islam. It was then that he changed his name to Muhammad Ali. In 1974 Ali regained his title after defeating the former champion Joe Frazier and the then-current champion George Foreman. He lost to Leon Spinks in 1978 but later that year regained the title a third time, becoming the first heavyweight champion ever to do so. He retired in 1979, having lost only three of 59 fights. Attempted comebacks in 1980 and 1981 failed. Throughout his career Ali was known for his aggressive charm, invincible attitude, and colourful boasts, often expressed in doggerel verse. I am the greatest was his personal credo. Ali's later years have been marked by physical decline. Damage to his brain, caused by blows to the head, has resulted in slurred speech, slowed movement, and other symptoms of Parkinson disease.