( CNN ) Boxing legend Muhammad Ali was released from a hospital Tuesday night after an 18-day stay for treatment of a urinary tract infection , a spokesman said Wednesday . Ali , 72 , was admitted to an undisclosed hospital on December 20 for what spokesman Bob Gunnell initially said was a mild case of pneumonia . The diagnosis later changed . `` After further observation and testing , doctors determined that Ali was suffering from a severe urinary tract infection , not pneumonia as originally diagnosed , '' Gunnell said Wednesday . `` The boxing legend has fully recovered and is now at his home . '' He said Ali was looking forward to celebrating his 73rd birthday on January 17 with his family and friends . Born Cassius Clay , the fighter won an Olympic gold medal as a light-heavyweight at 18 . In 1964 , he became world heavyweight champion in an upset victory against Sonny Liston . Shortly thereafter , he changed his name to Muhammad Ali to reflect his conversion to Islam . The boxer was also known for his protest against the Vietnam War and refusal to be drafted into service out of religious conviction . He retired from boxing in 1981 and announced his diagnosis with Parkinson 's disease three years later . CNN 's Chuck Johnston and Ben Brumfield contributed to this report .
Former world heavyweight champion had urinary tract infection , spokesman says . The former champ was hospitalized in January after he complained of a severe bladder infection.