Participants were 118 juvenile offenders who met DSM-III-R criteria for substance abuse or dependence, along with their families. A multisource measurement battery was used to assess drug use, criminal activity, family relations, peer relations, school functioning, and out-of-home placements. Findings show that comorbidity for externalizing disorders was associated with high rates of antisocial behavior and predicted worse 16-month outcomes than substance abuse alone or substance abuse with comorbid internalizing disorders. For criminal activity and drug use, the presence of internalizing disorders buffered the deleterious effect of externalizing disorders on substance-abusing and substance-dependent juvenile offenders. Thus, even in substance-abusing delinquents, a population already extreme in antisocial behavior, the presence of externalizing disorders indicates high risk for deterioration. For substance-abusing adolescents with externalizing disorders, treatment should probably focus on enhancing parental competence to structure, monitor, supervise, and provide consequences for their adolescent's behavior. The key is to identify those aspects of the family and its social ecology that are barriers to effective parental discipline. 5 tables and 24 references