The effect of Probucol on serum lipids, lipoproteins and the apoproteins A1, A2 and B was studied in 27 patients with primary hypercholesterolemia. After 3 months of dietary treatment and a 6 week placebo period the patients received 4 X 250 mg of Probucol or placebo per day for 4 months in a double blind design. Total and LDL-cholesterol were significantly reduced with Probucol (13% and 16% respectively) in addition to the diet. Apo B showed a 12% decrease. The HDL cholesterol concentration as well as the serum triglycerides and triglyceride-rich lipoproteins were not significantly altered. Both apo A1 and apo A2 were markedly reduced under Probucol treatment. In general the subjective and objective tolerance of the drug was good.