Four hundred female sex workers attending a sexually transmitted disease clinic in Lima, Peru, were interviewed for demographic information and medical, contraceptive, and sexual practice histories. Cervical cultures were done for Neisseria gonorrhoeae and Chlamydia trachomatis, and serum was tested for antibodies to human immunodeficiency virus, human T cell lymphotropic virus type I (HTLV-I), Treponema pallidum, C. trachomatis, herpes simplex virus type 2 (HSV-2), and Haemophilus ducreyi. The prevalence of HTLV-I increased with duration of prostitution from 3.6% (6 years; P > .01). After adjustment for duration of prostitution, reduced risk of HTLV-I was significantly correlated with condom use for more than half of all sexual exposures for >3 years (odds ratio [OR], 0.34; 95% confidence interval [CI], 0.13–0.89). Further adjusting for condom use, HTLV-I seropositivity was associated with C. trachomatis (OR, 3.7; 95% CI, 1.4-13.2) and with antibody to HSV-2 (OR, 3.7; 95% CI, 0.5–29.6). Thus, duration of prostitution, lack of consistent condom use, and past infection with C. trachomatis were significantly associated with HTLV-I seropositivity.