Twelve patients with Type IV hyperlipoproteinemia were treated with clofibrate and halofenate in a double blind, crossover trial for two years. Drug intake was monitored by determination of the level of the drugs in serum. Halofenate and clofibrate were equally effective in lowering plasma triglycerides and cholesterol levels. Patients who were secretors of ABO blood group antigens in saliva had a greater hypocholesterolemic response to both drugs than those who were nonsecretors. Clofibrate treatment resulted in a significant rise of low density lipoprotein cholesterol. Both drugs lowered serum bilirubin levels and this effect had a significant positive correlation with that on uric acid levels. Halofenate had a greater hypouricemic effect than clofibrate and may be a useful drug for treatment of patients with Type IV hyperlipoproteinemia who have concomitant hyperuricemia.