The amount of cross-sectional area narrowing by atherosclerotic plaques was determined histologically in each 5 mm segment of the entire lengths of the right, left main, left anterior descending, and left circumflex coronary arteries in 40 patients with fatal coronary heart disease and known fasting serum total cholesterol and triglyceride levels. The patients were divided into four groups based upon the serum total cholesterol and triglyceride levels: group I, total cholesterol of 250 mg/dl or less, triglyceride of 170 mg/dl or less; group II, total cholesterol of 250 or less, triglyceride of more than 170; group III, total cholesterol of more than 250, triglyceride of 170 or less; group IV, total cholesterol of more than 250, triglyceride of more than 170. The number of 5 mm segments of coronary artery narrowed severely (76 to 100 percent in cross-sectional area) by atherosclerotic plaques in each group was as follows: 172 of 505 (34 percent) 5 mm segments from group I; 242 of 353 (69 percent) segments from group II; 120 of 295 (41 percent) from group III and 425 of 884 (48 percent) segments from group IV. The mean percentage of 5 mm segments narrowed severely was significantly greater in group II than in group I (p less than 0.005) or group III (p less than 0.01). Additionally, the mean number of four coronary arteries per subject severely narrowed and the number of subjects with severe narrowing of the left main coronary artery were significantly greater in groups II and III than in group I. The percentages of 5 mm segments narrowed severely correlated significantly with the serum triglyceride level (p less than 0.03). Although it correlated with the number of severely narrowed coronary arteries per subject, the serum total cholesterol level did not correlate with the percentage of 5 mm segments of coronary artery with severe narrowing.