This randomized, multicenter, double-blind parallel-group study was performed to evaluate the lipid-altering efficacy and safety of simvastatin 80 mg/day, a dose twice the current maximum recommended dose. At 20 centers in the United States, 521 male and female hypercholesterolemic patients were randomly assigned in a ratio of 2:3 to receive simvastatin 40 or 80 mg once daily, respectively, for 24 weeks in conjunction with a lipid-lowering diet. Patients met National Cholesterol Education Program (NCEP) low-density lipoprotein (LDL) cholesterol criteria for pharmacologic treatment. The mean percentage reductions (95% confidence intervals) from baseline in LDL cholesterol averaged at weeks 18 and 24 were 38% (-40 to -36) and 46% (-47 to -45) for the 40- and 80-mg groups, respectively (p <0.001 between groups). One third of patients on the 40- and 80-mg doses achieved an LDL cholesterol reduction of 46% and > or = 53%, respectively. Decreases in apolipoprotein B, total cholesterol, and triglycerides were also significantly greater among patients receiving 80 mg/day. Simvastatin was well tolerated in both groups. Two patients (0.6%) in the 80-mg group developed myopathy. Consecutive, clinically significant hepatic transaminase elevations occurred in 3 (1.0%) and 6 (1.9%) patients in the 40- and 80-mg groups, respectively (p= 0.486). In conclusion, simvastatin 80 mg/day provided substantial reductions in LDL cholesterol, allowing most patients to reach their NCEP target levels; it also had an excellent safety and tolerability profile.