This randomized, placebo-controlled, double-masked, parallel-group trial assessed the serum cholesterol-lowering effects of atorvastatin, a new 3-hydroxy-3-methylglutaryl coenzyme. A reductase inhibitor, over 26 weeks in patients with primary hypercholesterolemia. Thirty-nine patients from four centers in the United States were originally randomized to one of two treatment groups and received either atorvastatin 10 mg (20 patients) or placebo (19 patients) once daily. Atorvastatin rapidly and significantly reduced serum total cholesterol, low-density lipoprotein cholesterol (LDL-C), and apolipoprotein B levels. LDL-C was reduced 35% with atorvastatin 10 mg compared with a 0% increase in LDL-C in the placebo group. Atorvastatin significantly reduced triglyceride levels, with improvements occurring over time. At 26 weeks, triglyceride levels were reduced by 21% with atorvastatin treatment compared with a 14% increase with placebo. The drug was well tolerated and no clinically significant laboratory abnormalities were detected.