The protective efficacy of an oral inactivated whole cell Vibrio cholerae plus recombinant B subunit cholera vaccine was determined against El Tor cholera among Peruvian children and adults (2-65 years old) in a randomized, double-blind manner. Study subjects received 2 doses of vaccine or placebo 2 weeks apart, followed by a booster dose 10 months later. Surveillance for cholera was performed actively, with 2 visits per week to each household, and passively, at a local hospital. Stool samples were collected during diarrhea episodes and were cultured for V. cholerae. A total of 17,799 persons received 2 doses of vaccine or placebo, and 14,997 of these persons received the booster dose. After 2 doses (first surveillance period), V. cholerae biotype O1 was isolated from 17 vaccinees and 16 placebo recipients, demonstrating vaccine efficacy (VE) of -4%. After 3 doses (second surveillance period), V. cholerae O1 was isolated from 13 vaccinees and 32 placebo recipients, demonstrating VE of 61% (95% confidence interval ¿CI, 28%-79%). In the second surveillance period, the VE for illness requiring hospitalization was 82% (95% CI, 27%-96%). VE was also higher for persons >15 years old (VE, 72%; 95% CI, 28%-89%).