Background: Risk perceptions are theoretically important in predicting health behavior but have only shown modest evidence in predicting sexual risk behavior.Purpose: Our purpose is to investigate whether perceptions of the local prevalence of disease serve as a predictor of health behavior, particularly behaviors associated with infectious diseases such as HIV/AIDS.Methods: Four hundred eighty-seven men and 236 women receiving diagnostic and treatment services at a sexually transmitted infections (STI) clinic in a moderate-size U.S. city completed anonymous surveys of perceived prevalence of HIV/AIDS and other STI and sexual risk and protective behaviors.Results: Participants who estimated a lower AIDS burden in their city relative to other U.S. cities demonstrated greater numbers of sex partners, higher rates of sexual risk practices, and higher rates of STI. They were also less likely to have been tested for HIV.Conclusions: This initial study of intuitive epidemiology suggests that STI clinic patients may have a sense for the relative burden of AIDS in their city and estimates of local disease prevalence may predict sexual risk behaviors.