This multicenter study evaluated the efficacy and safety of simvastatin in patients with primary hypercholesterolemia and in patients with hypercholesterolemia associated with type II diabetes who were being treated with diet or oral antidiabetic agents. The metabolic control of diabetes during drug treatment was also evaluated. Of 90 patients from seven centers, 88 (48 nondiabetic and 40 diabetic) patients completed the study; 2 patients dropped out for reasons other than drug-related adverse events. Patients were admitted to the study if their total cholesterol was greater-than-or-equal-to 250 mg/dl, low-density lipoprotein (LDL) cholesterol was between 160 to 350 mg/dl, and total triglycerides were <350 mg/dl. Patients followed an isocaloric diet during the entire trial. After a 4-week placebo period, treatment with simvastatin was started at 10 mg/day; the daily dose was increased to 20 mg after 6 weeks if the total cholesterol values were still >200 mg/dl and again from 20 to 40 mg after 12 or 18 weeks if the goal cholesterol value (<200 mg/dl) was not achieved; active drug therapy lasted 24 weeks. At the end of the treatment, compared with baseline, total and LDL cholesterol decreased by 29.7% and 39.1%, respectively, in the patients with primary hypercholesterolemia (mean daily dose, 30.2 mg) and by 30.3% and 42.8%, respectively, in type II diabetic patients (mean daily dose, 21.1 mg). A significant increase in high-density lipoprotein cholesterol was observed in both groups (9.3% in nondiabetic and 11.2% in diabetic patients). Safety parameters did not vary significantly during the trial; side effects were mild and rare. No variations in lens opacities were observed during the study. In diabetic subjects, mean fasting and postprandial glucose, as well as hemoglobin A1c, did not change during the trial. Simvastatin is an effective and safe drug not only for the treatment of primary hypercholesterolemia but also for the treatment of hypercholesterotemia associated with type II diabetes.