To evaluate whether bacterial vaginosis predicts the acquisition of sexually transmitted diseases (STDs), we studied 255 nonpregnant female subjects aged 15-30 who reported recent sexual contact with a male partner in whom either gonococcal or chlamydial urethritis or nongonococcal urethritis was diagnosed. Compared to subjects with normal vaginal flora, subjects with bacterial vaginosis were more likely to test positive for Neisseria gonorrhoeae (odds ratio [OR], 4.1; 95% confidence interval [CI], 1.7-9.7) and Chlamydia trachomatis (OR, 3.4; 95% CI, 1.5-7.8). Subjects colonized vaginally by hydrogen peroxide-producing lactobacilli were less likely to receive a diagnosis of chlamydial infection or gonorrhea than subjects without such lactobacilli. Bacterial vaginosis was a strong predictor of gonorrhea and chlamydial infection among subjects who reported recent exposure to a male partner with urethritis. These data support the importance of vaginal flora in the defense against STD acquisition.