Different laboratory and metabolic studies were used to obtain an etiologic classification of hyperlipidemia in 286 patients with atherosclerosis. The studies showed that although time-consuming metabolic studies were required to clearly establish the nature of the basic disturbance, useful information could usually be obtained by serum lipoprotein analysis (paper electrophoresis) and serum lipid determinations (cholesterol, phospholipid, and triglyceride concentrations). More than 90% of the 286 patients were found to have hyperglyceridemia derived from increased endogenous lipogenesis from carbohydrate. This abnormal carbohydrate sensitivity was revealed with an ad libitum carbohydrate (35% to 40%) diet. Since lipoproteins synthesized from carbohydrates were shown to be rich in both triglyceride and cholesterol, carbohydrate-sensitive hyperglyceridemia was frequently found in association with hypercholesteremia. The abnormal metabolism was controlled by a sugar-free diet, with a carbohydrate allowance of 125 to 150 gm supplied as starches.