We conducted a prospective study among women in Mombasa, Kenya, to determine whether Trichomonas vaginalis infection was associated with an increased risk of human immunodeficiency virus type 1 (HIV-1) infection. At monthly follow-up visits, laboratory screening for HIV-1 and genital tract infections was conducted. Among 1335 HIV-1-seronegative women monitored for a median of 566 days, there were 806 incident T. vaginalis infections (23.6/100 person-years), and 265 women seroconverted to HIV-1 (7.7/100 person-years). Trichomoniasis was associated with a 1.52-fold (95% confidence interval, 1.04-2.24-fold) increased risk of HIV-1 acquisition after adjustment for potential confounding factors. Treatment and prevention of T. vaginalis infection could reduce HIV-1 risk in women.