Adolescents with insulin-dependent diabetes mellitus (IDDM) face increasing responsibilities for managing their own treatment. For some, implementing their treatment regimen enhances diabetes selfefficacy beliefs because they welcome the chance to exert control over their illness. Other adolescent patients, however, feel overwhelmed and helpless. We developed the self-efficacy for diabetes scale (SED) with a sample of adolescent boys (n = 34) and girls (n = 34) with IDDM. High reliability (internal consistency) and evidence for criterion validity were obtained for this measure, because SED scores predicted metabolic control. In addition, construct validity was established, because SED scores were related to theoretically relevant measures of locus of control and self-esteem. Although they showed similar expectancies for diabetes self-efficacy, girls had significantly positive correlations between their SED scores and metabolic control, whereas boys did not. We offer a new instrument for studying adolescent patient perceptions and physical health.