Analysis of the lipid and lipoprotein levels in the blood of 23 hyperlipoproteinemic patients with xanthomata, permitted the separation of three clinical types on the basis of the % β-lipids. These corresponded to types II, III, and IV of Fredrickson. This separation is important because type IV and to a lesser degree III are carbohydrate induced and improve rapidly on a weight reducing diet and carbohydrate restriction. The level of triglycerides in the plasma is the most helpful information for rapid differentiation of cases of primary hyperlipidemia: (less than 350 mg/100 cc in type II, more than 350 mg/100 cc in types I, III, IV, or V). Serum cholesterol determination is not of further aid, because it is elevated in types II, III, and IV. The very rare types I and V can be excluded by the absence of excess quantities of chylomicrons. Final distinction between types III and IV is accomplished by determining the percentage of pre-β-lipids. Hyperbeta-lipoproteinemia (type II) is characterized clinically by xanthoma tendineum and xanthelasma palpebrarum. In hyperpre-β-lipoproteinemia (type IV), xanthoma striatum palmare (or xanthochromia), and xanthoma papuloeruptivum are characteristic. Type III has clinical findings of both type II and type IV. Xanthoma tuberosum is found with equal frequency on all three types. Types II and III are frequently complicated by atherosclerotic heart disease.