The present study tested the effects of older siblings' outcome expectancies, health risk behaviors, and consequences on the behavior and health-related expectancies of their younger adolescent siblings. Data were analyzed for 140 matched pairs of younger (n = 147) and older (n = 195) siblings. Younger siblings' alcohol use and expectancies were significantly associated with perceptions of their older siblings' drinking. Younger siblings' positive expectancies for other health risk behaviors (e.g., sex without a condom) were associated with their perceptions about the positive consequences their older sibling had experienced and with their older siblings' positive expectancies. These results suggest that vicarious learning from an older sibling is one mechanism through which adolescents form expectancies about health risk behaviors. Prevention strategies are discussed that focus on expectancies and older sibling influence on adolescent risk involvement.