Although Trichomonas vaginalis is a common sexually transmitted pathogen, the significance and natural history of trichomoniasis remain undefined in the male patient. We conducted a longitudinal study to examine the relationship of T. vaginalis to nongonococcal urethritis in men and to increase our understanding of the natural history of this infection. As previously reported, T. vaginalis was isolated from 50 of 447 men attending a sexually transmitted disease clinic by culture of urethra, first-void urine or external genitalia. Semen cultures proved valuable for documentation in select cases, including 4 instances when concomitant cultures of the external genitalia, urethra and first-void urine sediment were all negative. Spontaneous resolution was documented in 36% of untreated men but 1 asymptomatic man had persistence of T. vaginalis throughout a 4-month period. Nongonococcal nonchlamydial urethritis was documented in 12 of 21 men (57%) at the visit before treatment or spontaneous resolution compared to only 2 (10%) after elimination of T. vaginalis (p < 0.001). These findings suggest that spontaneous resolution of trichomoniasis and prolonged asymptomatic carriage occur in men with trichomoniasis and that T. vaginalis is a treatable cause of urethritis among sexually active men.