Trichomoniasis is probably the most widespread sexually transmitted disease of man. The causative organism, Trichomonas vaginalis, can normally be found on microscopic examination of vaginal exudate, although serological diagnostic techniques have recently been described. The organism has now been recognized as a pathogen, and, although infection has been associated with cervical cancer, a causative link has not been established. Despite both humoral and cellular immune responses, protective immunity has not been demonstrated in man. Treatment schedules with metronidazole and other 5-nitro-imidazoles have been revised, many authors recommending short courses with large doses. The suspected carcinogenic properties of metronidazole have not been proved, although caution is still advised in the treatment of pregnant patients. Recognition of trichomoniasis as a sexually transmitted disease means that treatment of sexual partners is essential to prevent reinfection.