A randomized, double-blind 12-week comparison of lovastatin and gemfibrozil in the treatment of patients with primary hypercholesterolemia with normal or moderately elevated triglycerides was performed in 334 patients from 19 centers in Finland. Patients with "high" total serum cholesterol (240 to 300 mg/dl) constituted Stratum 1 and patients with "very high" total serum cholesterol (greater than 300 mg/dl) constituted Stratum 2. In Stratum 1, patients were randomly assigned to either lovastatin 20 mg nightly or gemfibrozil 600 mg twice daily, and in Stratum 2 to either lovastatin 40 mg nightly or gemfibrozil 600 mg twice daily. In both strata, the lovastatin dose was doubled after 6 weeks if serum cholesterol remained greater than 200 mg/dl. Ninety-two and 93% of the patients doubled their dose in Strata 1 and 2, respectively, resulting in average doses of 38.5 mg/day (Stratum 1) and 77.4 mg/day (Stratum 1) and 77.4 mg/day (Stratum 2) by week 12. The dose of gemifibrozil was kept constant. Lovastatin reduced low-density lipoprotein (LDL) cholesterol by 31 and 42% in Stratum 1 and 2, respectively. The corresponding reductions achieved by gemfibrozil were 13 and 18%. In both strata, as well as in patients with Type IIa and IIb hyperlipoproteinemia, lovastatin was approximately 2 to 4 times as effective as gemfibrozil in lowering LDL cholesterol. Although both drugs increased high-density lipoprotein (HDL) cholesterol concentrations, gemfibrozil was 1.5 to 3 times more effective. LDL/HDL cholesterol ratios decreased significantly more during lovastatin therapy. Both drugs reduced serum triglyceride levels, but gemfibrozil was much more effective.(ABSTRACT TRUNCATED AT 250 WORDS)