The efficacy and tolerability of lovastatin and gemfibrozil were compared in a randomized double-blind 12-week study including 182 patients with primary hypercholesterolemia, from 7 hospitals in Spain. Inclusion criteria were total-cholesterol of at least 250 mg/dl and triglycerides less than 350 mg/dl. Patients were stratified in two groups: group 1, cholesterol less than 300 mg/dl, and group II, cholesterol equal to or more than 300 mg/dl. Patients were randomized to gemfibrozil (600 mg b.i.d.) or lovastatin (20 mg q.p.m., group I and 40 mg q.p.m., group II). If after 6 weeks of treatment cholesterol remained above 200 mg/dl, lovastatin does were doubled. In group I, lovastatin decreased cholesterol by 20%, LDL-C by 28%, and triglycerides by 17%, and increased HDL-C by 8%. In group II the results were: -26%, -33%, -19% and +6% respectively. The corresponding results with gemfibrozil were: -8%, -9%, -28% and +14% (group I); and -13%, -14%, -33% and +9% (group II). In both groups, lovastatin was more effective in reducing cholesterol and LDL-C (P less than 0.001) and gemfibrozil in reducing triglycerides (P less than 0.05 group I and P less than 0.01 group II). Both drugs were well tolerated. Thus, lovastatin and gemfibrozil are effective lipid-lowering agents; lovastatin has more pronounced effects in patients with hypercholesterolemia.