To examine the long-term effects of a brief family intervention on aggressive and hostile behaviors of adolescents in the general population. Randomized trial including 22 public schools assigned to the Iowa Strengthening Families Program or a control condition. Analyses supported sample representativeness and failed to show differential attrition effects 4 years after baseline. Seven-session intervention for parents and their sixth-grade children. The multi-informant, multimethod measures included independent observer ratings of adolescent aggressive and hostile behaviors in adolescent-parent interactions, family-member report of aggressive and hostile behaviors in those interactions, and adolescent self-report of aggressive and destructive conduct across settings. Data were collected during the 6th (preintervention and postintervention), 7th, 8th, and 10th grades. All measures showed a generally positive trend in intervention-control group differences over time. During 10th grade, significant intervention-control differences were found for adolescent self-report of aggressive and destructive conduct (P =. 01), with relative reduction rates ranging from 31.7% to 77.0%. Significant differences were shown for observer-rated aggressive and hostile behaviors in adolescent-parent interactions (P =.01); differences in family member reports of those behaviors were not significant. Supplemental analyses of both interactional behavior measures, specific to parent sex, indicated significant experimental group differences in interactions with mothers (P =.04 for both measures) but not with fathers. Brief family competency-training interventions designed for general populations can reduce aggressive and hostile behaviors in adolescents' interactions with parents and adolescent aggressive behaviors outside of the home setting. Thus, this type of intervention has important public health implications. Arch Pediatr Adolesc Med. 2000;154:1248-1257.