A 5-year longitudinal study of African American adolescents, aged 10 to 12 at Time 1, used the prototype/willingness (prototype) model to examine the (social) cognitive effects of the onset of sexual behavior on self-concept. Structural equation modeling (SEM) showed that becoming sexually active was related to favorable changes in adolescents' self-concepts and that this effect was moderated by gender. The effect was more pronounced among boys than girls. Positive self-concept, in turn, was related to subsequent risky sexual behavior. Sexual onset was also associated with positive changes in adolescents' images of the typical adolescent who has sex (i.e., sex prototype). This increase in prototype favorability marginally predicted subsequent willingness to have risky sex. In sum, sexual debut was related to increases in adolescents' self-concepts and risk cognitions, both of which predicted risky sexual behavior. (Contains 1 figure and 2 tables.)