We analyzed the potential effects of different levels of performance on eight Health Care Employer Data and Information Set (HEDIS) measures for cardiovascular disease and diabetes during 1995-2005. The measures targeted 3.3 million (25 percent) heart attacks. Improvements in performance to those achieved by the median plan in 2005 imply prevention of 1.9 million myocardial infarctions (MIs, 15 percent), 0.8 million strokes (8 percent), and 0.1 million cases of end-stage renal disease (17 percent). If performance had been 100 percent, 1.4 million more MIs would have been prevented. Control of blood pressure has the largest potential effect on quality at the national level.