This article reports the impact of a multicomponent quality improvement intervention on adherence with 13 measures of diabetes care and a summary measure, the Diabetes Summary Quality Index (Diabetes-SQUID). The intervention was conducted between January 1, 2004, and July 1, 2005, within 66 primary care practices in 33 states, including 372 providers and 24 250 adult patients with diabetes. Across all practices, the average Diabetes-SQUID was 50.6% (10th percentile 36.5%, 90th percentile 63.0%) on January 1, 2004, and 58.4% (10th percentile 47.6%, 90th percentile 69.7%) on July 1, 2005, with an average absolute improvement of 7.8% (95% confidence interval, 5.9%-9.7%). Significant improvements occurred for 12 of the 13 individual measures: blood pressure and urine microalbumin monitoring; HDL cholesterol, LDL cholesterol, triglyceride, and glycosylated hemoglobin measurements; prescription of antiplatelet therapy; and blood pressure, HDL-cholesterol, LDL-cholesterol, triglyceride, and glycosylated hemoglobin control. The findings suggest that a multicomponent intervention can have a robust impact on quality of care for diabetes.