To assess the cause of nonspecific vaginitis, we performed a prospective case-control study of vaginal flora and a randomized unblinded trial of different therapies. Haemophilus vaginalis was isolated from 17 to 18 women with signs of vaginitis but only one of 18 normal matched controls (P less than 0.002). The concentration of anaerobic bacteria in vaginal washings also was increased in patients. Clinical improvement and eradication of H. vaginalis occurred in one of seven patients given sulfonamide vaginal cream, two of 15 given oral doxycycline, nine of 27 given oral ampicillin, and 80 of 81 given oral metronidazole. On the seventh day of therapy signs of nonspecific vaginitis persisted in 31 of 31 with, and in two of 92 without, persistent H. vaginalis infection (P less than 0.001). These data suggest the causal role of H. vaginalis in nonspecific vaginitis, possibly in concert with vaginal anaerobes. The widespread use of sulfonamide creams is inappropriate. Metronidazole is effective, but its efficacy must be weighed against its possible toxicity.