This randomized, double-blind evaluation of metronidazole therapy for trichomonal vaginitis compared the efficacy and side effects of a single 2-g dose and the standard seven-day regimen (250 mg three thimes daily). Eighty (86%) of the 93 women examined seven to 21 days after therapy with the 2-g regimen, and 76 (91.6%) of 83 examined after the seven-day regimen, were cured. These cure rates were not significantly different. In addition, symptom duration and the occurrence of side effects and yeast infection were not significantly different for the two treatment groups. Because the efficacy and side effects of the two regimens are comparable and the 2-g dose is easier to administer and less expensive, we recommend the 2-g dose as standard treatment for trichomoniasis.