OBJECTIVE To compare the safety and efficacy of lovastatin and simvastatin in patients with primary hypercholesterolemia. METHODS Fourteen Canadian centres participated in this double-blind, randomized, parallel-design study with a six-week screening period, a four-week placebo baseline period and an 18-week active treatment period. Patients were included in the study if their total cholesterol (TC) was at least 6.2 mmol/L and total triglycerides (TG) were 4.0 mmol/L or less at baseline. Half of the patients were in stratum I (TC 6.2 to 7.8 mmol/L at baseline and placebo period) and half in stratum II (TC greater than 7.8 mmol/L). The initial dose of lovastatin or simvastatin (20 and 10 mg/day, respectively) was doubled if the patient's cholesterol was greater than 5.2 mmol/L after six and/or 12 weeks, to a maximum of 80 mg/day lovastatin or 40 mg/day simvastatin. Of 298 randomized patients, two had baseline data only (and were excluded from the efficacy analysis), while 77 were treated with lovastatin and 74 with simvastatin in stratum I, and 72 were on lovastatin and 75 on simvastatin in stratum II. RESULTS In stratum I, both lovastatin and simvastatin lowered TC (-26.0% in both the lovastatin and simvastatin groups), low density lipoprotein (LDL) cholesterol (-33.4% in lovastatin and -34.4% in simvastatin), TG (-11.4% in lovastatin and -16.2% in simvastatin), apolipoprotein (apo)-B (-24.8% in lovastatin and -26.3% in simvastatin) and the TC:high density lipoprotein (HDL) cholesterol ratio (from 6.65 to 4.73 in lovastatin and from 6.45 to 4.46 in simvastatin), and increased HDL cholesterol (+3.6% in lovastatin and +7.8% in simvastatin) and apo-A1 (+6.3% in lovastatin and +9.0% in simvastatin) with P < 0.001 in all within-group tests except for HDL cholesterol (P < 0.05). Similar results were obtained in stratum II for TC (-30.7% in lovastatin and -30.3% in simvastatin), LDL cholesterol (-37.6% in lovastatin and -36.8% in simvastatin), TG (-21.9% in lovastatin and -16.9% in simvastatin), apo-B (-32.0% in lovastatin and -31.7% in simvastatin), TC:HDL cholesterol ratio (from 8.62 to 5.47 in lovastatin and from 8.96 to 5.77 in simvastatin), HDL cholesterol (+9.7% in lovastatin and +7.5% in simvastatin) and apo-A1 (+7.2% in lovastatin and +8.8% in simvastatin), with P < 0.001 in all within-group tests. Serious adverse events (clinical and laboratory) were reported in four patients in the lovastatin group and three in the simvastatin group. The most reported nonserious adverse effects were gastrointestinal tract (15 patients in the lovastatin group and 16 in the simvastatin group) and musculoskeletal (14 patients in the lovastatin group and 11 in the simvastatin group). Medication was withdrawn in eight patients. CONCLUSIONS Both lovastatin and simvastatin were found to be effective and well tolerated in each stratum. However, there were no significant differences between lovastatin and simvastatin in the treatment of moderate or severe primary hypercholesterolemia.