Context.-Medicare has a legislative mandate for quality assurance, but the effectiveness of its population-based quality improvement programs has been difficult to establish.Objective.-To improve the quality of care for Medicare patients with acute myocardial infarction.Design.-Quality improvement project with baseline measurement, feedback, remeasurement, and comparison samples.Setting.-All acute care hospitals in the United States.Patients.-Preintervention and postintervention samples included all Medicare patients in Alabama, Connecticut, Iowa, and Wisconsin discharged with principal diagnoses of acute myocardial infarctions during 2 periods,