In subjects with trichomoniasis the number of Trichomonas vaginalis in vaginal discharges or secretions is unknown. The presence of T. vaginalis was evaluated in 177 consecutive female patients attending an inner city sexually transmitted disease clinic by patient history, wet mount, and broth culture. T. vaginalis was quantitated by a novel agar culture technique. Of the 177 women, 86 (49%) were positive for T. vaginalis by either wet mount or culture. Clinical symptoms were not reliable for making an accurate diagnosis of trichomoniasis. Culture on modified Diamond's medium was more sensitive (98%) than the wet mount method (38%) in detecting T. vaginalis. Of the 86 patients who were positive for trichomoniasis, quantitation was obtained for 81 patients, with 70% yielding greater than 10(4) colony-forming units (cfu)/ml. The number of T. vaginalis ranged from 40 to greater than 10(6) cfu/ml. The wet mount method was very insensitive for detecting T. vaginalis and was positive only in patients yielding greater than 10(5) cfu/ml.