Ruiz has been able to build a record of 39-5-1 with 27 knockouts.
Ruiz has gained a lot of critizism in part because of the way he won the Heavyweight championship. After champion Lennox Lewis beat Evander Holyfield to unify the crown, the World Boxing Association (WBA) demanded him to defend vs Ruiz. Lewis deemed Ruiz as 'non-deserving' and chose to leave the WBA belt vacant. After this, the WBA designated Ruiz and Holyfield to fight for the vacant crown, and Holyfield won on a controversial 12 round split decision. Because of the closeness of the bout, the WBA ordered an immediate rematch, and in a spectacular bout, Ruiz came back from having a swollen eye and dropped Holyfield in the 11th round, securing a unanimous decision and the realization of his dream: Becoming the first Hispanic Heavyweight champion of the world.
Ruiz defended the title twice, with a draw in a 3rd bout with Holyfield, and a 10 round disqualification win vs Canada's Kirk Johnson. Talks about a fight vs Mike Tyson were stalled because of a court decision ruling that because of contractual problems, they can't fight for the moment. And, as for Lewis, he is still declining to face Ruiz inside a ring.
On March 1 of 2003, Ruiz lost the title, by a 12 round unanimous decision, to Roy Jones Jr.
On October 10 of that same year, it was announced that Ruiz would substitute for former rival David Tua (who beat Ruiz by knockout in 19 seconds in 1996) in a fight against former world Heavyweight champion Hasim Rahman for the interim WBA heavyweight title on December 13. Ruiz was the winner of that fight by a twelve round unanimous decision, and he now will have the right to challenge Jones at a later date, or if Jones fails to defend the heavyweight title, will inherit it from him.