[1] Viloria went to the Sydney Olympics as a medal favorite, but lost to eventual gold medalist Brahim Asloum of France, 6-4.

On September 10, 2005, Viloria dropped down to light flyweight and won the WBC crown by knocking out Eric Ortiz in the first round with a powerful right.

Romero was then stripped of his title after failing a post fight drug test for methamphetamine and the decision was overturned into a no-contest.

On April 14, 2007, he fought Edgar Sosa for the vacant WBC Light Flyweight title but lost via majority decision.

Viloria then decided to move up to the super flyweight division and began training under former champion, Robert Garcia, at La Colonia Gym in Oxnard, California.

[3] Brian Viloria won via unanimous decision against Jose Garcia Bernal (26-11-1) in his debut at the super flyweight division at the Alameda Swap Meet in Los Angeles, California on January 5, 2008.

[10] Viloria returned to the flyweight division against Mexico's Omar Soto on July 10, 2010, at the Yñares Sports Arena, Pasig, Metro Manila.

[11] On July 16, 2011, Viloria captured the WBO Flyweight title by besting Mexican Julio César Miranda.

[13] On May 13, 2012, Viloria stopped his rival Omar Niño Romero in the 9th round for another successful defense of his WBO Flyweight title.