The indirect hemagglutination (IHA) technique was evaluated for use in the serologic study of infection with Trichomonas vaginalis. The IHA test showed that sera from 88% of women attending a venereal disease clinic had antibody to T. vaginalis. The antibody frequency and titers were highest in women who had documented infections due to T. vaginalis. Serologic and cultural evidence of recent or active trichomonal infection was found in 11% of 85 men who had nongonococcal urethritis, but was absent in a control group of 27 men. Clinical findings in ten men with symptomatic genitourinary trichomoniasis are described; all but one had relatively high (greater than or equal to 1:80) IHA antibody titers. Antibody to T. vaginalis was found significantly (P less than 0.005) more often in sera from women than in sera from men in an apparently healthy group of individuals between the ages of 1 and 20 years. The IHA test appears potentially useful for the diagnosis of trichomoniasis in men and in the seroepidemiology of infections due to T. vaginalis.