UFC 31 was the event that adopted the current standard weight classes under the Unified Rules of Mixed Martial Arts.
UFC 31: Locked and Loaded took place on May 4, 2001, at the Mark G. Etess Arena in Atlantic City, New Jersey. The event followed the New Jersey State Athletic Control Board’s adoption of the Unified Rules in April 2001, and the UFC used the card to introduce the reformed divisions.
The changes included raising the former 150-pound lightweight limit to 155 pounds, while setting welterweight at 170, middleweight at 185, and light heavyweight at 205. These divisions created the familiar structure that governs most modern UFC matchups. UFC 30 came immediately before the change, which is why it is sometimes mistaken for the milestone event.
The card also featured Randy Couture defending the heavyweight title against Pedro Rizzo, Pat Miletich facing Carlos Newton for the welterweight championship, Matt Serra’s UFC debut, and B.J. Penn’s professional debut. UFC 31 therefore marked both a rules transition and a notable generation of new talent.