Objective: The purpose of this study was to determine whether treatment of bacterial vaginosis (BV) decreases the incidence of sexually transmitted diseases (STDs). Study Design: Women with asymptomatic BV were studied prospectively to determine the effect of treatment of BV for the prevention of STD. Women were assigned randomly to observation or treatment and prophylaxis with intravaginal metronidazole gel. Women were screened monthly for STDs. Results: Women who were assigned randomly to receive metronidazole gel had a significantly longer time to the development of STDs compared with women in the observation group (P = .02). The 6-month STD rate was 1.58 per person-year (95% CI, 1.29, 1.87) for women in the metronidazole gel group vs 2.29 per person-year (95% CI, 1.95, 2.63) for women in the observational group. The difference in STD rates was driven by a significant difference in the number of chlamydial infections (P = .013). Conclusion: Treatment and twice-weekly prophylactic use of intravaginal metronidazole gel resulted in significantly fewer cases of chlamydia. © 2007 Mosby, Inc. All rights reserved.