Using both cross-sectional and longitudinal data spanning a decade, this article analyzes how dietary behavior of household children and adults (N = 576 households) was affected by the Stanford Five-City Project (FCP). Tests of a three-part, cumulative model of bidirectional influences within the family, which conceptualizes household members as sources of influence on each other and subject to influence by an external agent, were supported. Children and adults were influenced by both each other and the FCP campaign in changing and maintaining health behaviors. This article demonstrates that public health campaigns can be made more effective if they conceptualize both children and adults as potential sources of influence. The long-term effectiveness of such efforts can be enhanced by encouraging families, as opposed to individuals, to change health behaviors.