Background: In Perú, a previous study of government-sponsored periodic examinations of female sex workers (FSWs) found no significant impact on rates of sexually transmitted diseases (STDs). Goal: This study assessed the impact of technically improved periodic STD services on STD prevalence and on consistent condom use (CCU). Study Design: The study involved monthly follow-up of 917 FSWs at two STD clinics, with evaluations before and during implementation of strengthened STD services, which included periodic screening and treatment for gonorrhea, chlamydial infection, trichomoniasis, bacterial vaginosis (BV), and syphilis; counseling; and supply of condoms. Outcome analyses were adjusted for participation bias. Results: During 7908 person-months of observation, the prevalences of gonorrhea, chlamydial infection, trichomoniasis, and BV declined significantly and CCU increased significantly, with similar trends in both clinics. During follow-up, gonorrhea, chlamydial infection, trichomoniasis, and BV were negatively associated with follow-up after screening. BV also was positively associated with use of an intrauterine device and negatively with douching. CCU during follow-up was associated with significantly decreased risk of gonorrhea, chlamydial infection, and trichomoniasis. Conclusions: Strengthened periodic screening for and treatment of confirmed STD, in addition to condom promotion and provision, represent feasible, effective interventions in commercial sex, and time series analyses can provide a useful approach to evaluating new interventions.