OBJECTIVE - This study reports 6- and 12-month follow-up for the families of adolescents with diabetes who participated in a trial of Behavioral-Family Systems Therapy (BFST). RESEARCH DESIGN AND METHODS - A total of 119 families of adolescents with type 1 diabetes were randomized to 3 months of treatment with either BFST, an education and support (ES) group, or current therapy (CT). Family relationships, adjustment to diabetes, treatment adherence, and diabetic control were assessed at baseline, after 3 months of treatment, and 6 and 12 months later. This report focuses on the latter two evaluations. RESULTS - Compared with CT and ES, BFST yielded lasting improvements in parent-adolescent relationships and diabetes-specific conflict. Delayed effects on treatment adherence emerged at 6- and 12-month follow-ups. There were no immediate or delayed effects on adolescents' adjustment to diabetes or diabetic control. CONCLUSIONS - BFST yielded lasting improvement in parent-adolescent relationships and delayed improvement in treatment adherence, but it had no effect on adjustment to diabetes or diabetic control. A variety of adaptations to BFST could enhance its impact on diabetes outcomes.