To assess factors associated with having a Trichomonas vaginalis (TV) infection among persons receiving care for human immunodeficiency virus (HIV) and estimate the number of transmitted HIV infections attributable to TV.HIV clinic patients were recruited from 2 secondary prevention studies, screened by urine nucleic-acid amplification tests for sexually transmitted infections, and interviewed about risk factors (baseline, 6, and 12 months). We conducted mathematical modeling of the results to estimate the number of transmitted HIV infections attributable to TV among a cohort of HIV-infected patients receiving medical care in North Carolina.TV was prevalent in 7.4%, and incident in 2% to 3% of subjects at follow-up. Individuals with HIV RNA <400 copies/mL (odds ratio, 0.32; 95% CI: 0.14-0.73) and at least 13 years of education (odds ratio, 0.24; 95% CI: 0.08-0.70) were less likely to have TV. Mathematical modeling predicted that 0.062 HIV transmission events occur per 100 HIV-infected women in the absence of TV infection and 0.076 HIV infections per 100 HIV- and TV-infected women (estimate range: 0.070-0.079), indicating that 23% of the HIV transmission events from HIV-infected women may be attributable to TV infection when 22% of women are coinfected with TV.The data suggest the need for improved diagnosis of TV infection and suggest that HIV-infected women in medical care may be appropriate targets for enhanced testing and treatment.