Acute myocardial infarctions were generally believed to result from plaque rupture and thrombosis at the site of a "mild to moderate" coronary stenosis. To assess the severity of coronary stenoses that predisposed to acute coronary syndrome, the 317 patients prospectively included were (1) 102 patients with acute ST-elevation myocardial infarction (STEMI) referred for primary percutaneous coronary intervention (PCI), (2) 135 patients with non-STEMI or unstable angina pectoris (UAP) referred for semiurgent PCI, and (3) 80 patients with stable angina pectoris (SAP) admitted for elective PCI. Patients with STEMI were included if thrombus aspiration could restore normal antegrade coronary blood flow. After aspiration (but before PCI), a high-quality angiogram was obtained and the reference diameter, minimal luminal diameter, and percentage of diameter stenosis of the culprit lesion were quantified. In patients with non-STEMI/UAP and SAP, aspiration was not performed. Average diameter of stenosis was similar in patients with STEMI and those with SAP (66 +/- 12% vs 65 +/- 10%, respectively; p = NS), but was slightly larger in patients with non-STEMI/UAP (71 +/- 12%; p <0.05 vs both STEMI and SAP). In patients with STEMI, only 11% of culprit stenoses were found to have diameter stenosis <50% after removal of the thrombus. In conclusion, most STEMIs occurred at the site of severe coronary stenosis. Diameter stenosis severity was <50% in a minority of cases.