OBJECTIVE: To assess the impact of a quality improvement (QI) intervention on the quality of diabetes care at primary care clinics. RESEARCH DESIGN AND METHODS: Twelve primary care medical practices were matched by size and location and randomized to intervention or control conditions. Intervention clinic staff were trained in a seven-step QI change process to improve diabetes care. Surveys and medical record reviews of 754 patients, surveys of 329 clinic staff, interviews with clinic leaders, and analysis of training session videotapes evaluated compliance with and impact of the intervention. Mixed-model nested analyses compared differences in the quality of diabetes care before and after intervention. RESULTS: All intervention clinics completed at least six steps of the seven-step QI change process in an 18-month period and, compared with control clinics, had broader staff participation in QI activities (P = 0.04), used patient registries more often (P = 0.03), and had better test rates for HbA(1c) (A1C), LDL, and blood pressure (P = 0.02). Other processes of diabetes care were unchanged. The intervention did not improve A1C (P = 0.54), LDL (P = 0.46), or blood pressure (P = 0.69) levels or a composite of these outcomes (P = 0.35). CONCLUSIONS: This QI change process was successfully implemented but failed to improve A1C, LDL, or blood pressure levels. Data suggest that to be successful, such a QI change process should direct more attention to specific clinical actions, such as drug intensification and patient activation.