We investigated long-term hypolipidemic effects and clinical safety of simvastatin, a new competitive inhibitor of 3-hydroxy-methylglutaryl coenzyme A reductase in 24 patients with familial and non-familial hypercholesterolemia. Patients received up to 40 mg simvastatin for a period of 30 months. Significant decreases were noted in plasma cholesterol (30%), plasma triglycerides (25%), very low density lipoprotein-cholesterol (26%), and low density lipoprotein-cholesterol (40%), whereas an increase in plasma high density lipoprotein-cholesterol (11%) was observed. Furthermore, the percentage decrease in plasma low density lipoprotein cholesterol was independent of individual baseline concentrations. Simvastatin did not alter the composition of low density lipoproteins or high density lipoproteins. The percentage decrease in total plasma and low density lipoprotein-cholesterol was independent of apoprotein E isoforms and low density lipoprotein-receptor activity as assayed in cultured fibroblasts. The drug therapy was well tolerated and clinical examinations revealed no adverse effects. Clinical chemistry indices and hematological, as well as endocrinological parameters remained within normal limits and ranges.