The histological characteristics of acute myocardial infarction were examined in specimens obtained from infarct-related coronary artery lesions (20 left anterior descending artery, 2 left circumflex artery, 8 right coronary artery) in 30 patients with initial acute myocardial infarction who underwent directional coronary atherectomy following intracoronary thrombolysis within 6 hours after the onset of chest pain. Resected tissues were fixed in 10% buffered formalin and embedded in paraffin, and the 4 microns-thick paraffin sections were stained with hematoxylin-eosin and examined by light microscopy. Thrombus and/ or intramural hemorrhage were present in all samples. There were high incidences of cholesterol cleft in 19 (63%), foam cell in 21 (70%), calcium deposit in 19 (63%) and intimal proliferation in 16 (53%). These data suggest that thrombus and/or intramural hemorrhage are important in the onset of acute myocardial infarction.