Causal models of adolescent substance abuse from a family systems perspective are developed using data from a large-scale family therapy efficacy grant. It is argued that the literature on families of adolescent substance abusers is scattered in its theoretical and empirical efforts, tends to not account for individual and family developmental influences on adolescents' behavior, is laden with poor instrumentation, and tends to treat all drugs indiscriminately. In an attempt to address these concerns, a structural equation modeling approach incorporating systemic and developmental (individuation) perspectives on drug abuse is used in examining the impact of the family system on overall drug use severity, and tobacco, alcohol and marijuana use. The sample included 111 families of adolescent substance abusers representing families of youthful drug users typically seen in treatment. Overall, the family systems causal models explained significant proportions of variance in adolescent illicit drug use, but were not predictive of licit drug use; none of the family systems variables were predictive of the frequency of adolescent tobacco or alcohol use. Conversely, greater family cohesiveness and open family communication were negatively related to overall drug use severity and marijuana use. Democratic parenting styles were associated with greater marijuana use. Finally, there was a trend suggesting that clearer intergenerational roles predict less adolescent marijuana use. Propositions based on the findings are developed and directions for intervention-prompted changes in family functioning on adolescent drug use are offered. (Twelve tables, 9 figures, and 77 references are included). (Author)