In 4 subsets of patients with coronary artery disease, the amounts of narrowing of the 4 major epicardial coronary arteries were compared (left main, left anterior descending, left circumflex and right) by atherosclerotic plaques. Among 129 patients studied at necropsy, an average of 2.7 of the 4 arteries were narrowed greater than 75% in cross-sectional area at some point; in control subjects, narrowing was seen in an average of 0.7 arteries. Patients with unstable angina pectoris had a greater incidence of narrowing (3.2 arteries) than did patients with sudden coronary death (2.8), acute myocardial infarction (MI) (2.7) or healed MI (2.3). Each of the 4 major arteries was divided into segments 5 mm in length, and histologic sections were prepared and stained by the Movat method. A total of 6,461 segments were analyzed from the 129 patients and 1,849 from the 40 controls. In the 129 patients, 35% of the 5-mm segments were narrowed 75 to 100% in cross-sectional area (compared with 3% in control subjects). The group with unstable angina had the highest percentage (48%) of severely narrowed segments compared with the groups with sudden coronary death (36%), acute (34%) and healed MI (31%). Only 8% of the 6,461 segments were narrowed less than or equal to 25% in cross-sectional area, and virtually none of the 6,461 segments was normal; thus, 92% of the coronary segments were narrowed greater than 25% in cross-sectional area by atherosclerotic plaque alone. Among patients with fatal coronary artery disease studied at necropsy, therefore, the atherosclerotic process is severe and diffuse in the major epicardial coronary arteries.