The amount of cross-sectional area (XSA) narrowing in each 5 mm long segment of each of the four major epicardial coronary arteries was determined in each of five patients with type III hyperlipoproteinemia (HLP) and symptomatic, fatal atherosclerotic coronary disease (CAD). Four had angina pectoris; two had acute myocardial infarcts which healed, and two died suddenly. Of the four major epicardial coronary arteries, all four were narrowed 76% to 100% in XSA by atherosclerotic plaques in two patients, three were narrowed to this degree in two patients, and two were so narrowed in one patient. Three patients had severe narrowing of the left main coronary artery. The percent of 5 mm long segments of coronary artery narrowed to various degrees was as follows: 96% to 100%, 0 to 37 (mean 14); 76% to 95%, 14 to 61 (mean 35); 51% to 75%, 9 to 41 (mean 24); 26% to 50%, 0 to 42 (mean 16), and 0% to 25%, 0 to 27 (mean 11). Utilizing a scoring system of 1 to 4 for the four categories of narrowing (1 = 0% to 25%, 2 = 26% to 50%, 3 = 51% to 75% and 4 = 76% to 100% XSA narrowing), scores per 5 mm segment for each patient ranged from 2.5 to 3.9 (mean 3.1). Thus these five type III HLP patients had severe diffuse coronary narrowing by atherosclerotic plaques.