Peer antisocial behavior robustly predicts adolescents' own behavior, but not all adolescents are equally vulnerable to their peers' influence and genetic factors may confer vulnerability. This study used data of = 3,081 adolescents from the Avon Longitudinal Study of Parents and Children to examine whether brain-derived neurotrophic factor (BDNF), a polymorphism that affects psychological functioning, moderates the association between affiliation with aggressive peers at age 10 and own aggression at age 15. A significant gene–environment interaction was found, where those who affiliated with aggressive peers in childhood showed increased risk of being aggressive in adolescence if they carried the BDNF met-met variant compared with val-val carriers. Our findings underline the importance of both biological and social factors for adolescent development.