The efficacy on plasma lipids, apo A1 and B of a HMG Co A Reductase inhibitor, simvastatin, and a fibrate derivative, gemfibrozil, were compared in 136 hypercholesterolemic patients. The study was randomized, double-blinded and the active drug was given after a 4 week period of placebo. Gemfibrozil (n = 69) was given at 900 mg q.p.m. during the entire study. The primary dose of simvastatin was 10 mg q.p.m. during the first 6-weeks of treatment. At the end of this period, the dose was doubled if the cholesterol level was above 2 g/l (5,16 mmol/l). The same modification was carried out 12 weeks after the beginning active drug treatment with the same criteria. The baselines of total cholesterol were 3.24 +/- 0.69 g/l (8.38 +/- 1.78 mmol/l) 3.21 +/- 0.72 g/l (8.31 +/- 1.87 mmol/l) for patients with simvastatin and those with gemfibrozil respectively. So, at the end of active treatment, 64% of patients in the simvastatin group (n = 67) received 40 mg q.p.m. After 18 weeks of treatment, 89% of patients treated with Simvastatin and 37% with gemfibrozil had more than 20% reduction in LDL cholesterol. Simvastatin was more efficient on total cholesterol, LDL-C and apo B. The increase of HDL-C is similar in both groups of patients. In contrast, the level of triglycerides was further decreased by gemfibrozil. The tolerance was good in the two groups of patients and no difference in the frequency of side effects was observed.