Background. Vaginal infections are common, frequently recur, and may increase women's risk for sexually transmitted infections (STIs). We tested the efficacy of a novel regimen to prevent recurrent vaginal infections. Methods. Human immunodeficiency virus (HIV)-negative women 18-45 years old with 1 or more vaginal infections, including bacterial vaginosis (BV), vulvovaginal candidiasis (VVC), or Trichomonas vaginalis (TV), were randomly assigned to receive vaginal suppositories containing metronidazole 750 mg plus miconazole 200 mg or matching placebo for 5 consecutive nights each month for 12 months. Primary endpoints, evaluated every 2 months, were BV (Gram stain) and VVC (positive wet mount and culture). Results. Participants (N = 234) were randomly assigned to the intervention (N = 118) or placebo (N = 116) arm. Two hundred seventeen (93%) women completed an end-of-study evaluation. The intervention reduced the proportion of visits with BV compared to placebo (21.2% vs 32.5%; relative risk [RR] 0.65, 95% confidence interval [CI]. 48-.87). In contrast, the proportion of visits with VVC was similar in the intervention (10.4%) versus placebo (11.3%) arms (RR 0.92, 95% CI. 62-1.37). Conclusions. Monthly treatment with intravaginal metronidazole plus miconazole reduced the proportion of visits with BV during 12 months of follow-up. Further study will be important to determine whether this intervention can reduce women's risk of STIs.