BACKGROUND Social-psychological theories of health behavior suggest that adolescents' sexual behaviors are influenced by the sexual attitudes and behaviors of their friends. METHODS Data on 2,436 participants in the National Longitudinal Study of Adolescent Health (Add Health) who were sexually inexperienced at Wave 1 (1994-1995) were analyzed to examine whether friend-related variables predicted initiation of vaginal intercourse by Wave 2 (1996). Analyses also assessed whether predictive relationships varied by level of involvement with friends. Odds ratios were generated by logistic regression analysis. RESULTS In the 9-18 months between Waves 1 and 2, 18% of participants initiated intercourse. In analyses controlling for gender, family structure and romantic relationships, the higher the proportion of a youth's friends who were sexually experienced, the greater the odds of sexual debut (odds ratio, 1.01); the odds also were elevated among youth who believed that they would gain their friends' respect by having sex (1.2). Relationships between friend variables and sexual initiation did not vary by level of involvement with friends. CONCLUSIONS To maximize the likelihood of success, programs focused on delaying teenage sexual intercourse should address norms for sexual behavior among adolescents' close friends as well as the perceptions, skills and behaviors of individual youth.