The efficacy of simvastatin, a new inhibitor of cholesterol biosynthesis, was studied in patients with familial hypercholesterolemia. A mean reduction in low-density lipoprotein cholesterol of 38 percent was observed after eight weeks of treatment with 20 mg of simvastatin; a 42 percent reduction was seen after the next 16 weeks with 40 mg. There was no difference in response whether the drug was taken in one or two doses. In another study, simvastatin (40 mg per day) was compared with cholestyramine (8 to 16 g per day). After 12 weeks, low-density lipoprotein cholesterol was reduced 43 percent by simvastatin and 29 percent by cholestyramine. Combination therapy caused a reduction in low-density lipoprotein cholesterol of 54 percent. Simvastatin appeared also to be a potent hypolipidemic drug in 10 patients with familial dysbetalipoproteinemia. The drug was well tolerated. Adverse effects were primarily composed of increases in serum transaminases and creatine phosphokinase. No effects of simvastatin on the production of steroid hormones were observed.