Vaginal discharge of mixed etiology occurs frequently, with abnormal vaginal flora being the most common condition. The interrelationships among the disturbance of the vaginal ecology, the presence of yeasts, and infection with Trichomonas vaginalis and human immunodeficiency virus type 1 (HIV-1) were investigated among women presenting to a sexually transmitted diseases service. Analysis was done for 598 women. Although the prevalence of HIV-1 infection increased linearly with increasing Nugent's score (bacterial vaginosis score of Gram stain), the prevalence of T. vaginalis increased suddenly, from 12% in patients with a Nugent's score of < or =3 to 33% in patients with a score of 4, and remained at this level at higher scores. Yeast colonization and vulvovaginal candidiasis were inversely related to Nugent's scores. T. vaginalis might be responsible for the change in normal vaginal flora and may, therefore, be one of the causes of bacterial vaginosis. This could lead to more effective HIV-1 acquisition.