Over the past three decades, most research on adolescent sexual behavior has focused on vaginal intercourse and related behaviors, including contraception and unintended pregnancy. In this study, we describe the prevalence and correlates of vaginal, oral, and anal sex in an epidemiologically defined population in Baltimore, Maryland. Young adults (ages 18-24), who had been enrolled in a behavioral intervention trial during elementary school, were interviewed by telephone between 1998 and 2002 to assess their sexual behavior. Of 1679 respondents interviewed, 70.8% were Black and 55% were women. Overall, 93% of the young adults reported vaginal intercourse, 78% reported receiving oral sex, 57% reported performing oral sex, and 10% reported receptive anal intercourse. Among men, 27% reported insertive anal intercourse. Blacks initiated vaginal intercourse at an earlier age than Whites; White women performed oral sex earlier than Black women. Significant interactions were observed between age of first vaginal partner and both gender and race/ethnicity. Blacks with older partners initiated sex at an earlier age than both Blacks with a partner the same age or younger and Whites. We also observed a relationship between older female sex partners and earlier vaginal sex initiation among men. We conclude that older sex partners play an important role in sexual initiation among young adults. In light of the rates of oral and anal sex, sexual education and intervention programs should address the risk for unintended consequences of these behaviors.