Simvastatin, a potent inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, was compared with cholestyramine resin in a randomized open-label 12-week multicenter study of 251 high-risk patients with familial or nonfamilial hypercholesterolemia. Simvastatin, 20 mg and 40 mg daily, produced mean reductions in total cholesterol of 26% and 33%, respectively, and reductions in low-density lipoprotein cholesterol level of 32% and 40%. Cholestyramine resin, 4 to 12 g twice daily, reduced total cholesterol and low-density lipoprotein cholesterol levels 15% and 21%, respectively. High-density lipoprotein cholesterol levels were increased 8% to 10% by all treatments. Plasma triglyceride levels were moderately decreased by simvastatin treatment, while triglyceride levels increased with cholestyramine treatment. Simvastatin was better tolerated than cholestyramine, which had numerous gastrointestinal tract side effects. No patient had a serious drug-related adverse event.