Simvastatin and bezafibrate actions on blood lipids and their side effects were compared in a double-blind trial involving 24 adults with severe type IIa or IIb primary hypercholesterolemia (mean plasma cholesterol = 4.35 g/l). During a 12-week period, the patients received either bezafibrate, 600 mg 3 times a day, or simvastatin, 10 or 20 mg once a day, with a doubling of the dosage at week 6 if the LDL-cholesterol level remained above 1.40 g/l. Simvastatin significantly reduced LDL-cholesterol by -39.5% (p less than 0.001), total cholesterol by 33.9% (p less than 0.005) and apoprotein B by -28% (p less than 0.001). Bezafibrate significantly reduced LDL-cholesterol (-19.8%, p less than 0.001) and total cholesterol (-17.5%, p less than 0.002), but not apoprotein B. Bezafibrate also reduced triglycerides by -26.6% and raised HDL-cholesterol by +27.6%. Simvastatin was more effective than bezafibrate in lowering LDL-cholesterol (p less than 0.002), total cholesterol (p less than 0.005) and apoprotein B (p less than 0.05). Tolerance of both drugs was considered excellent.