Background: Studies of sexual behavior and of interventions designed to reduce human immunodeficiency virus risk usually depend on self-report. Validation of self-reported condom use measures has not been previously reported in an urban population at high risk for sexually transmitted diseases and human immunodeficiency virus. Methods: A prospective cohort study was performed in subjects recruited from sexually transmitted disease clinics in Baltimore. At enrollment, a questionnaire was administered that assessed human immunodeficiency virus risk factors and sexually transmitted disease history, and used a retrospective calendar to assess sexual events and condom use over the previous 30 days. Clinical evaluation was performed for sexually transmitted diseases. At fol-low-up 3 months later, the same procedures were repeated. Incident sexually transmitted diseases at follow-up were defined as new culture or serologically documented diagnoses of gonorrhea, chlamydia, syphilis, or trichomoniasis. Results: In the 323 male and 275 female (total = 598) subjects who completed a follow-up visit, 21% reported using condoms for every act of sexual intercourse over the previous 30 days, 21% reported occasionally using condoms, and 59% reported not using condoms. At follow-up, 21% of subjects had new incident gonorrhea, chlamydia, syphilis, or trichomoniasis. Fifteen percent of the men who were “always” condom users had incident sexually transmitted diseases compared with 15.3% of “never users;” 23.5% of women who were “always” users had incident sexually transmitted diseases compared with 26.8% of “never” users. Conclusions: In this high-risk population, self-reported condom use is not associated with lower sexually transmitted disease incidence. This finding suggests that self-reported condom use measures, even in a research setting, may be subject to substantial reporting bias. © 1995 American Sexually Transmitted Diseases Association.