The effects of simvastatin (MK-733), a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, on serum lipids, lipoproteins, and apolipoproteins were investigated in 29 patients (12 men, 17 women, aged 37 to 73) with moderate to severe hypercholesterolemia. It was given in doses of 2.5 mg/day for four months and 5 mg/day for the succeeding four months. Total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and apolipoprotein (apo) B decreased by 18% (263 +/- 7 mg/dl to 216 +/- 7 mg/dl, P less than 0.01), 24% (180 +/- 7 mg/dl to 136 +/- 7 mg/dl, P less than 0.01), and 21% (133 +/- 4 mg/dl to 104 +/- 3 mg/dl, P less than 0.01), respectively, four months after treatment. Similar reductions (17%, 24%, and 23%, respectively, P less than 0.01) were observed at eight months. A significant reduction in triglyceride (TG) was observed (173 +/- 15 mg/dl to 136 +/- 11 mg/dl at eight months, P less than 0.01), as was a significant increase in serum high-density lipoprotein cholesterol (HDL-C) (48 +/- 2 mg/dl to 52 +/- 2 mg/dl at eight months, P less than 0.01). However, apo AI and apo AII remained unchanged. Atherogenic indices of (TC--HDL-C)/ HDL-C, LDL-C/HDL-C, and apo B/Apo AI ratios were significantly (P less than 0.01) reduced after treatment. No significant changes were observed in lipoprotein lipase, hepatic TG lipase, and lecithin: cholesterol acyltransferase (LCAT) activities. Simvastatin was well tolerated and no critical side effects were noted in the eight-month study period. These data indicate that simvastatin, even at a low dose of 2.5 to 5 mg daily, causes consistent reductions in serum TC, LDL-C, apo B, and TG, and a rise in HDL-C and antiatherogenic lipoproteins.