Study Objective: To evaluate the efficacy and tolerability of lovastatin under controlled conditions in heterozygous familial hypercholesterolemia. Design: Randomized, double-blind, placebo-controlled, multicenter trial. Setting: Five lipid clinics with a central laboratory and coordinating center. Patients: 101 adult patients with heterozygous familial hypercholesterolemia. Interventions: Patients were on a lipid-lowering diet throughout the study. After a 4-week placebo baseline period, patients were randomized to five equal treatment groups. Each group received a different sequence of placebo or lovastatin 5 to 40 mg twice daily or 20 to 40 mg once daily in the evening, during three consecutive 6-week periods. Measurements and Main Results: The mean reductions in total plasma cholesterol and low-density lipoprotein cholesterol across the dosage ranges were 14% to 34% and 17% to 39%, respectively (p compared with zero and placebo < 0.01). High-density lipoprotein cholesterol and apolipoproteins Al and All rose slightly. Apolipoprotein B fell substantially at the higher dosage levels (-23% at 40 mg twice daily, p < 0.01), indicating a reduction in the concentration of circulating low-density lipoprotein particles. Maximum response was achieved in 4 to 6 weeks. Twice-daily dosing was slightly more efficient than once-daily dosing. Of those patients receiving 40 mg twice a day, 89% had a fall in low-density lipoprotein cholesterol of at least 20%, and 61% had a fall of at least 40%. Adverse effects attributable to lovastatin were minimal, and no patient was withdrawn from the study. Conclusion: Lovastatin was well tolerated and effective in the treatment of familial hypercholesterolemia.