The effect of procetofen (fenofibrate) on lipids and apoproteins was studied in ten patients affected with familial type II-A and II-B hyperlipoproteinemia. Following stabilization with an isocaloric diet for 2 months, the patients were given 300 mg/day of procetofen for 3 months, then placebo for 1 month. Plasma cholesterol and triglycerides were decreased by 15% and 46%, respectively. Plasma apo B was lowered by 17%. The predominant decrease in lipids and apoproteins occurred in VLDL, whereas in HDL cholesterol increased significantly but not its major apoprotein, apo A-I. A mechanism of action involving increased catabolism of VLDL and reduced synthesis of LDL and VLDL is proposed.