OBJECTIVE: To assess the social-cognitive, behavioral, and physiological outcomes of a self-management intervention for youth with type 1 diabetes. RESEARCH DESIGN AND METHODS: A total of 81 youth with type 1 diabetes aged 11-16 years were randomized to usual care versus a "diabetes personal trainer" intervention, consisting of six self-monitoring, goal-setting, and problem-solving sessions with trained nonprofessionals. Assessments were completed at baseline and multiple follow-up intervals. A1C data were obtained from medical records. ANCOVA adjusting for age and baseline values were conducted for each outcome. RESULTS: At both short-term and 1-year follow-up, there was a trend for an overall intervention effect on A1C (short-term F = 3.71, P = 0.06; 1-year F = 3.79, P = 0.06) and a significant intervention-by-age interaction, indicating a great effect among older than younger youth (short-term F = 4.78, P = 0.03; 1-year F = 4.53, P = 0.04). Subgroup analyses demonstrated no treatment group difference among younger youth but a significant difference among the older youth. No treatment group differences in parent or youth report of adherence were observed. CONCLUSIONS: The diabetes personal trainer intervention demonstrated significant effects in A1C among middle adolescents.