Background and Objectives: A survey was conducted to determine the sexual behavior practices and prevalence of HIV-1, HTLV-I/II, and T. pallidum infections among unlicensed female sex workers (FSWs) in Lima, Peru. Goal of this Study: To assess the role of unlicensed FSWs as a potential source of retroviral and T. pallidum infection. Study Design: Female sex workers from 15 brothels were enrolled. Sera samples were obtained and tested for antibodies to HIV-1, HTLV-I, and Treponema pallidum. Results: Of 158 FSWs studied, all were negative for HIV-1, 6 were positive for HTLV-I, and 5 were positive for Treponema pallidum. Of their male clients, 75% used condoms, whereas only 3% reported condom use with their steady partners. When condoms were always used by clients, the history of a sexually transmitted disease was significantly reduced (p < 0.01), and the prevalence of HTLV-I (p < 0.05) and syphilis was lower among these workers. Conclusion: Data suggested that the low rate of sexually transmitted diseases among FSWs reflected the high rate of condom use by their male clients.