The clinical and biochemical features of type III hyperlipoproteinemia are described in 49 patients from 23 to 70 years of age. An increase in very low-density lipoproteins (VLDL) of abnormal chemical composition was the basis for diagnosis. The untreated patients all had hypercholesterolemia and hyperglyceridemia, and, on the average, decreased concentrations of both low- and high-density lipoproteins. Seventy-four percent had xanthomas, and classic "xanthoma striata palmaris" was found in more than half. Twenty-seven percent had ischemic heart disease, detected earlier in men than in women. Twenty-seven percent had peripheral vascular disease (compared to 4% of subjects with type II hyperlipoproteinemia). Twenty-five of 35 subjects achieved normal lipid levels with dietary therapy alone. Analysis of 29 kindred showed hyperlipidemia in half of adult blood relatives; half of these had type III, the remainder usually had sample endogenous hyperglyceridemia (type IV). Only 2 of 55 children less than 20 years of age were affected, both with type IV.