This is a report of a 43-year-old man (case 1) and a 61-year-old woman (case 2) with familial hyperlipoproteinemia type III. Both patients had cutaneous xanthomas, including palmar involvement and an increase in plasma cholesterol and triglycerides. Electrophoresis of plasma lipoproteins revealed a "broad β-band" blending with the pre-β area. Preparative ultracentrifugation at density 1.006 revealed a large fraction of the "β-material" floating in the supernate (D < 1.006). Fifty to sixty percent of the plasma cholesterol was found in the very low density lipoprotein fraction. Case 1 responded well to a low-fat diet and clofibrate. Case 2 did not respond to low-fat, low-carbohydrate, and low-cholesterol diets, and also failed to respond to a low-carbohydratephenformin regime. A combined low-carbohydrate diet and conjugated-equine estrogen regime resulted in control of the hyperlipidemia and disappearance of cutaneous xanthomas. The hyperlipidemia also responded well to a low-carbohydrate-clofibrate regime.