Data from 1,087 adolescent participants in three waves of the National Longitudinal Study of Adolescent Health were used to examine the effects of peer selection and socialization processes in adolescence on later reports of sexually transmitted infections (STI) and unintended pregnancies. Friends' attitudes and behavior were assessed with friends' reports. Among male adolescents, there was evidence for selection effects on STI diagnoses and socialization effects on reports of unintended pregnancy, both involving friends' attitudes. Among female adolescents, there was evidence for long-term effects of both socialization and selection processes involving same-sex friends' attitudes. Discussion focuses on the importance of peer and individual attitudes as potential intervention targets. (Contains 3 figures and 3 tables.)