Coronary heart disease is an important cause of death in patients with non-insulin-dependent diabetes mellitus (NIDDM) and is particularly common in diabetic populations that have relatively high levels of plasma cholesterol. To determine whether plasma cholesterol levels in patients with NIDDM could be reduced by drug therapy, we assessed the effect of lovastatin, a potent inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, in a randomized double-blind placebo-controlled manner in 16 white patients with NIDDM and mild to moderate elevations of plasma cholesterol. Lovastatin (20 mg twice daily) or a placebo was given for four weeks, during which blood glucose concentrations remained controlled. As compared with the placebo, lovastatin reduced total cholesterol by 26 percent, low-density lipoprotein (LDL) cholesterol by 28 percent, and LDL apolipoprotein B by 26 percent. Lovastatin therapy also reduced plasma triglycerides and very-low-density lipoprotein cholesterol by 31 percent and 42 percent, respectively. Although there was no change in the plasma level of high-density lipoprotein (HDL) cholesterol, the ratio of total cholesterol to HDL cholesterol fell by 29 percent. No side effects or abnormalities in serum values were noted during short-term lovastatin therapy. The beneficial effects of lovastatin on plasma lipid levels in patients with NIDDM could decrease the risk of the development of coronary heart disease.