The aims of the present study were to count Trichomonas vaginalis organisms recovered from the vaginas of patients with trichomoniasis, and to obtain data concerning changes in sizes of trichomonal populations during the menstrual cycle. In about 80% of symptomatic as well as symptomless patients, more than 1 x 10(5) parasites per ml could be obtained from vaginal washes. During menstruation, however, the number of trichomonads decreased appreciably, with subsequent increases within three to six days after bleeding. The results indicated that sufficient numbers of fresh trichomonads may be obtained from vaginal washes for biochemical and molecular experiments and also confirm the previously reported trichomonocidal effect of menstrual blood complement in vivo.