The efficacy, safety profile, and tolerability of the HMG-CoA reductase inhibitors simvastatin and lovastatin were compared in a multicenter, randomized, double-blind study in patients with moderate hypercholesterolemia. Commonly prescribed doses of these two drugs were used by 544 men and women, who followed an American Heart Association phase I diet during a 6-week baseline period and for the 24 weeks of active treatment. Simvastatin 10 mg and lovastatin 20 mg produced statistically significant reductions in total and low-density lipoprotein cholesterol (LDL-C). Patients receiving simvastatin 10 mg once daily and lovastatin 20 mg once daily experienced similar reductions in LDL-C and total cholesterol; however, simvastatin 20 mg was statistically superior to lovastatin 40 mg in decreasing these lipid fractions. For all treatment groups, increases in high-density lipoprotein cholesterol were inversely related to baseline levels. Moderate decreases in triglycerides occurred with all doses. Lipoprotein(a) levels, measured in a subset of patients, were similar before and after treatment. Both drugs were well tolerated.