Atherosclerotic plaque rupture with superimposed thrombosis is recognized as the lesion causing greater than 90% of acute myocardial infarctions. To determine the severity of atherosclerosis at the site of plaque rupture, 184 coronary arteries from autopsies of 162 patients who died of acute myocardial infarction were studied. There were 102 men, 72 +/- 10 years old (mean +/- SD), and 60 women, 75 +/- 8 years old. All arteries were dissected from the heart, fixed, decalcified, cut at 2 to 3 mm intervals and processed routinely for histologic examination. A planimeter was used to measure artery, plaque, thrombus and luminal cross-sectional area at the site of plaque rupture with thrombosis in sections projected at x13.8 magnification. At the site of atherosclerotic plaque rupture with superimposed thrombosis, the degree of stenosis due to plaque was: 90 +/- 7% for the right (n = 67), 91 +/- 6% for the left anterior descending (n = 79) and 91 +/- 6% for the left circumflex (n = 38) coronary arteries. Plaque rupture in fatal acute myocardial infarction occurs at sites of severe narrowing (mean 91%, range 67% to 99%). Thus, plaque rupture with thrombosis is unlikely to cause the fatal acute myocardial infarction in patients with mild to moderate coronary stenosis.