The amounts of narrowing of the 4 major (left main, left anterior descending, left circumflex and right) epicardial coronary arteries by atherosclerotic plaques were compared in 4 subsets of coronary patients. Of the 129 patients studied at necropsy, an average of 2.7 of the 4 arteries was narrowed greater than 75% in cross-sectional area at some point (0.7/4 in controls), and the group with unstable angina pectoris (3.2/4) had more narrowing than did the groups with sudden coronary death (2.8/4), acute myocardial infarction (2.7/4) and healed myocardial infarction (2.3/4). Each of the 4 major epicardial coronary arteries was divided into 5-mm long segments and a histologic section was prepared and stained by the Movat method of each of the 6,461 segments in the 129 patients and in the 1,849 segments in the 40 control subjects. In the 129 patients, 35% of the 5-mm segments were narrowed 75 to 100% in cross-sectional area (3% in controls) and the group with unstable angina had the highest percent (48%) of segments severely narrowed compared to the groups with sudden coronary death (36%), acute myocardial infarction (34%) and healed myocardial infarction (31%). Thus, of the 4 subsets of patients with fatal coronary artery disease studied at necropsy, those with unstable angina pectoris had the most severe and extensive coronary atherosclerosis.