Young adults residing in four States were enrolled in the period 1985-88 in a multicenter study of cardiovascular disease risk factors. In 1989, 2,729 members of the group were given a self-administered questionnaire that included questions on changes in sexual behavior that subjects had made in response to the epidemic of acquired immunodeficiency syndrome (AIDS). The final sample of 1,601 young, heterosexual, urban respondents included 412 white men, 568 white women, 224 black men, and 397 black women, all ages 21 to 40 years. Overall, nearly 50 percent of the sample reported having made at least one change in their sexual behavior in response to the AIDS epidemic to decrease their risk of becoming infected by the human immunodeficiency virus. The mean number of changes was 0.8 for white men, 1.1 for white women, 1.6 for black men, and 1.5 for black women. Change was reported more frequently by black respondents than white, with no significant sex differences. The categories of respondents reporting behavior changes were more often young, with a history of recreational drug use, with more sex partners, or having had anal intercourse. The most commonly reported behavior changes were reducing the number of sex partners and being more careful in partner selection. Of the 54 percent of respondents who did not report any change in sexual behavior, about 70 percent reported unprotected sex with more than one partner in the previous year. Significant sexual behavior change in response to the AIDS epidemic remains a goal for health education efforts.