OBJECTIVE - Personal models of diabetes have been shown to be proximal determinants of self-care behavior in adults with diabetes, both cross-sectionally and prospectively. This study set out to test the predictive utility of this approach in adolescents with diabetes. RESEARCH DESIGN AND METHODS - Participants were recruited from four regional hospitals in southern England (n = 54). They completed questionnaires assessing diabetes self-care, well-being, and personal models of diabetes (perceived impact, perceived seriousness, and short- and long-term treatment effectiveness) at baseline and 1-year follow-up. GHb assays performed as part of the patients' usual diabetes care were used to assess glycemic control. RESULTS - After controlling for baseline anxiety, change in perceived impact of diabetes prospectively predicted adolescents' anxiety (β = -0.21; t = -2.65; P < 0.01). After controlling for baseline dietary self-care, change in perceived effectiveness of the diabetes treatment regimen to control diabetes predicted dietary self-care (β = -0.39; t = -3.28; P < 0.0005). Poorer dietary self-care and being female were predictive of poorer glycemic control (r² = 0.29; F = 2.74; P < 0.005). CONCLUSIONS - This study provides further support for the role of personal models of illness in determining responses to illness. As adolescents take responsibility for the management of their diabetes, parents, clinicians, educators, and interventionists should consider these adolescents' beliefs about their diabetes and its treatment as key factors influencing self-care, emotional well-being, and glycemic control.