The effect of probucol, which is both a cholesterol-lowering drug and an antioxidant, on the serum concentrations of diet-derived antioxidants vitamin E, beta-carotene, lycopene, and vitamin A was studied in 303 hypercholesterolemic subjects. In a 3-year, double-blind, randomized trial we investigated to determine whether combined treatment with diet, cholestyramine, and probucol could reduce the progression of femoral atherosclerosis. Serum and lipoprotein antioxidant levels were measured by reverse-phase high-performance liquid chromatography. Cholestyramine significantly lowered serum concentrations of vitamin E by 7%, beta-carotene by 40%, and lycopene by 30% (all P < .001) due to impairment of gastrointestinal absorption and to serum cholesterol lowering. Probucol reduced serum vitamin E by 14% (P < .001) secondary to cholesterol and triglyceride lowering. The carotenoids were reduced by probucol by 30% to 40% (P < .001) most probably due to reductions in lipoprotein particle size and to competition with these substances for incorporation into VLDL during its assembly in the liver. This study shows that the use of a lipid-soluble antioxidant and cholesterol-lowering drug may have unfavorable effects on blood levels of diet-derived antioxidants.