Persons with human immunodeficiency virus (HIV) infection who subsequently develop an acute sexually transmitted disease have an increased probability of transmitting HIV. Therefore, characterizing such persons can help direct prevention efforts to a group who are likely to be continuing sources of HIV transmission. We assessed the incidence and factors associated with trichomoniasis in a cohort of HIV-infected women receiving care at a public clinic in Los Angeles County, California from 1992 through 1995. Demographic, clinical, and behavioral data were available from medical records and from patient interviews. Trichomonas infection was the most frequently identified sexually transmitted disease and was found in 37 (17.4%) of 212 women representing a crude incidence rate of 14.1 per 100 person-years experience. The crude rate of trichomoniasis was highest in black women (69.0 per 100 person-years), women with a history of trading sex for drugs or money (51.0 per 100 person-years), those using crack or cocaine (35.5 per 100 person-years), women with four or more sex partners (43.0 per 100 person years), and those born in the United States (23.3 per 100 person-years). Among women with severe immunosuppression (CD4+ count < 200), 18.4% (18 of 98) were diagnosed with trichomoniasis. After multivariate analysis using a Cox proportional hazards approach, black race (adjusted rate ratio [RR] = 5.6, 95% confidence interval [CI] = 2.3, 13.3) continued to be strongly associated with Trichomonas infection. Trading sex for money or drugs (adjusted RR = 25.2, 95% CI = 4.3, 148.6) and single marital status (adjusted RR = 3.7, 95% CI = 1.1, 13.0) were independent risk factors for trichomoniasis in nonblack women but not among black women. Data from this study indicate that Trichomonas may be a frequently acquired infection in HIV-positive women. Our findings suggest that HIV-infected women who are black, and nonblack women who trade sex for money or drugs or are unmarried, are at increased risk of trichomoniasis and therefore may be more likely to transmit HIV infection. Local HIV prevention strategies should target such women for intervention efforts.