The effects of the HMG-CoA reductase inhibitors lovastatin and pravastatin were studied over 4 months on serum lipids and lipoproteins in 35 patients with severe primary hypercholesterolaemia. In 17 patients 20 mg of lovastatin/day lowered the total cholesterol level by 18% (baseline 373 mg/dl) and LDL cholesterol by 20% (baseline 300 mg/dl). The corresponding data for 40 and 80 mg of lovastatin/day were respectively -23% and -29% for total cholesterol, and -30% and -36% for LDL cholesterol. Pravastatin at 20 mg/day lowered the total cholesterol in 18 patients by 20% (baseline 373 mg/dl) and LDL cholesterol by 24% (baseline 307 mg/dl). The corresponding data for 40 mg of pravastatin per day were 24% for total cholesterol and 30% for LDL cholesterol. So the effects of both HMG-CoA reductase inhibitors on total and LDL cholesterol are comparable. HDL (high-density lipoprotein) cholesterol was increased by lovastatin, whereas pravastatin showed no influence on HDL cholesterol. The reduction of serum triglycerides, VLDL triglycerides and VLDL cholesterol was more pronounced under treatment with lovastatin than under pravastatin.