Transactional models of problem behavior argue that less effective parenting and adolescent problem behaviors coevolve, exerting bidirectional influences. This article extends such models by analyzing growth trajectories of sexual risk behaviors and parenting processes among 3,206 adolescents (aged 13-18) and their residential parents. Within individuals, increases in regular family activities prospectively predicted declines in adolescents' risky sexual activities. In contrast, increases in risky sexual activities predicted heightened father knowledge. Between-individual comparisons revealed bidirectional links between more involved parenting, particularly family activities and father knowledge, and lower adolescent risky sexual activity. Results highlight the importance of family activities as a protective force for adolescents and suggest that fathers may react differently than mothers in the face of youth problem behaviors.