Aside from human papillomavirus, trichomoniasis is the most common sexually transmitted infection in the United States today, yet it has long been regarded as a sexually transmitted infection of minor importance. Medical opinion has traditionally held that it plays little role in health complications in women, and it is rarely seen in men. However, evidence has recently accumulated implicating Trichomonas vaginalis as a contributor to a variety of adverse outcomes among both sexes. Among both women and men, the association of T vaginalis with human immunodeficiency acquisition and transmission has been shown in multiple studies. Among women, trichomoniasis may play a role in development of cervical neoplasia, postoperative infections, and adverse pregnancy outcomes and as a factor in atypical pelvic inflammatory disease and infertility. Among men, trichomoniasis has emerged as a cause of nongonoccocal urethritis and as contributing to male factor infertility. As evidence continues to accumulate, the time has come to increase diagnostic efforts beyond traditional sexually transmitted disease clinic settings.