Hyperlipidemia associated with non-insulin-dependent diabetes mellitus (NIDDM) and insulin resistance is characterized by high triglyceride levels; raised levels of total low-density lipoprotein (LDL), which is made up of small, dense, cholesterol-rich particles; low levels of high-density lipoprotein (HDL); and glycosylation of apolipoproteins. Optimal drug therapy for this lipid profile is controversial. To test whether a fibrinic acid derivative (gemfibrozil) or a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor (lovastatin) would produce better results in these patients, a crossover study was performed. Gemfibrozil 600 mg twice daily and, after a washout period, lovastatin 20 to 40 mg twice daily were administered to nine patients with NIDDM. Gemfibrozil significantly decreased triglyceride, very-low-density lipoprotein (VLDL), and intermediate-density lipoprotein (IDL) levels, the total cholesterol:HDL ratio, and the IDL plus VLDL;HDL ratio, and significantly increased levels of HDL, HDL2, and HDL3. Lovastatin significantly decreased levels of total cholesterol, calculated LDL, directly measured LDL, IDL, total triglycerides, VLDL, and the ratios of LDL:HDL, total cholesterol:HDL, and directly measured LDL:HDL and significantly increased total HDL and HDL3 levels. Gemfibrozil was significantly more effective than lovastatin in raising total HDL and HDL3 levels and in lowering the IDL plus VLDL:HDL ratio. Lovastatin was significantly more effective than gemfibrozil in lowering total cholesterol, LDL, directly measured LDL, and the LDL:HDL and directly measured LDL:HDL ratios. In the absence of malignant hypertriglyceridemia, an HMG-CoA reductase inhibitor, rather than a fibrinic acid derivative, is indicated for the treatment of patients with dyslipidemia associated with NIDDM and insulin resistance.