This study investigated peer selection and socialization effects on sexual behavior and attitudes using 1,350 15- to 18-year-old students participating in two waves of the National Longitudinal Study of Adolescent Health. Regarding socialization effects, friends' intercourse without condoms predicted later individual intercourse without condoms positively. Friends' attitudes about the costs of sex predicted later individual attitudes positively and intercourse without condoms negatively. The latter relation was stronger for females than for males. Regarding selection effects, individual attitudes predicted later friends' attitudes positively, but the strength of this effect varied by ethnicity. The results suggest that adolescents socialize friends to have similar sexual attitudes and behavior but tend to select friends based on similar attitudes rather than similar behaviors.