Keywords: coronary artery; coronary plaque; ischemic heart disease Objectives: The objective of this study was to investigate the underlying stenosis severity of the culprit lesion in acute myocardial infarction. Background: It is widely believed that myocardial infarction often occurs in angiographically mild luminal stenosis. This, however, is in contradiction with experience from interventional practice in primary PCI. Methods: We performed quantitative coronary angiography (QCA) in 250 consecutive patients referred for acute percutaneous coronary intervention (PCI) because of acute myocardial infarction (AMI). Fundamental for analysis was that a realistic estimate of underlying luminal narrowing before the infarction could be made angiographically that QCA could be performed and that one of two criteria was met: (1) spontaneous reflow allowing assessment of the lumen proximal and distal to the culprit lesion, or (2) coronary artery closed at arrival but reflow after uncomplicated wiring allowing assessment of the lumen proximal and distal to the culprit lesion. Results: Of 250 consecutive patients (mean age 61.7 ± 12.7 years, 48 women) referred for acute PCI, 156 patients (62%) fulfilled at least one of the above criteria for reliable QCA. In 151 of these patients (96%) the severity of the underlying stenosis was >50% and in 103 (66%) it was >70%. There were no differences in stenosis severity between the left anterior descending [LAD, (72 ± 13)%, n = 57], left circumflex [Cx, (74 ± 10)%, n = 20], and right coronary artery territory [RCA, (74 ± 12)%, n = 76] (ANOVA, P = 0.76). There were no differences in stenosis severity between women [(73 ± 13)%, n = 36] and men [(75 ± 11)%, n = 120; P = 0.35]. Conclusion: In contrast to what is often believed, the majority of myocardial infarctions occurs in significant stenosis. © 2007 Wiley-Liss, Inc.