OBJECTIVE: To determine risk factors for sexually transmitted infections (STIs) among women in Durban and Hlabisa (South Africa), Moshi (Tanzania), and Lusaka (Zambia). STUDY DESIGN: Between 2003 and 2004, 958 women at risk of STIs were enrolled in a 1-year prospective study. They were interviewed at each monthly visit, and samples for STI testing were collected during quarterly and other visits when clinically indicated. RESULTS: The incidence of infections as measured in person-years at risk (PYAR) was as follows: overall trichomoniasis, 31.9/100 PYAR; chlamydial infection in South Africa, 19.5/100 PYAR; chlamydial infection in Tanzania and Zambia, 4.9/100 PYAR; gonorrhea in South Africa, 16.5/100 PYAR; gonorrhea in Tanzania and Zambia, 5.3/100 PYAR; overall syphilis, 7.5/100 PYAR; and overall HIV, 3.8/100 PYAR. The incidence of most STIs was highest among the South African sites, where chlamydial infection and gonorrhea were detected by using a more sensitive assay. Independent risk factors included age, hormonal contraceptive methods, and measures of sexual behavior, including number of sex partners and occurrence of anal sex in the past 3 months. Women with incident HIV infection were at increased risk of chlamydial infection [odds ratio (OR) = 5.5, 95% confidence interval (CI): 2.0-15.2]and gonorrhea (OR = 5.7, 95% CI: 1.9-17.0) in South African sites. Despite ongoing counseling during the study, high-risk sexual behaviors were common, and consistent condom use remained low. CONCLUSIONS: The incidence of STIs, including HIV, was high among women in this study. These findings highlight the urgent need for effective HIV/STI prevention programs in this population.