He had won the title on October 16, 1976 when he defeated the Filipino Ben Villaflor in the Hiram Bithorn Stadium of Puerto Rico, in a rematch ordered by the WBA president Elias Cordova.
Since that moment, Serrano made ten successful title defenses. First he fought the Ecuadorian Alberto Herrera, whom he defeated with a straight hand to the jaw in the 8 round. Then, he fought the Venezuelan Leonel Hernandez winning by unanimous decision in 15 rounds.
On August 27, 1977 he fought in the Roberto Clemente Coliseum of San Juan, making his third defense with Yoshio, whom he gave a boxing class in 15 rounds.
After Yoshio, Searrano dispatched Tae Ho Kim by TKO, followed by Mario Martínez, Young Ho Oh, Takao Maruki, Julio Valdez, Nkosana Mgxaji, Battlehawk Kazama.
Serrano lost his Crown with Yasutsune Uehara, on August 2, 1980. After 10 title defenses, Serrano touched the canvas and lost by KO for the first time in his career.
Serrano retired with a record of 52-4-1, 17 KO´s.