Metronidazole was given in various dosage regimens to 97 patients having microscopically diagnosed trichomoniasis. At the first examination after treatment all 97, including 76 to whom metronidazole had been given orally only, were found by culture and wet smear to be free of trichomonads. Reexamination of the 65 patients followed up for periods ranging from two weeks to 14 months revealed reappearance of trichomonads in eight cases. Nineteen husbands were treated. No patient had a recurrence after treatment of the husband. No effect of metronidazole on pregnancy or on fetal development was seen. Side effects, noted in 19 cases (20 per cent), generally were mild and transient and in no case were severe enough to terminate therapy before cure was effected.