This study investigated the associations between important dimensions of family relations and the metabolic control of adolescents with insulin-dependent diabetes mellitus (IDDM). Participants were 94 intact families (mother, father, and adolescent) of adolescents with IDDM. Metabolic control was determined by the adolescents' level of HbA(1c), and family relations were assessed with the Marital Adjustment Scale and the Family Adaptability and Cohesion Evaluation Scales-II. Although zero-order correlations showed that good metabolic control was associated with high family cohesion, family flexibility, and high marital satisfaction, hierarchical regression analyses revealed that these associations were considerably attenuated when participant demographic variables, especially the duration of IDDM, were controlled. Follow-up analyses showed that the associations between family relations and metabolic control were mediated by the duration of IDDM. Under conditions of short duration, there were strong associations between family relations and metabolic control. As duration lengthened, however, these associations decreased substantially. These results suggest that the associations between family relations and health are relatively complex in this sample of adolescents with IDDM.