The efficacy and safety of ezetimibe, a new cholesterol absorption inhibitor, was evaluated in this randomized, double-blind, placebo-controlled trial of 892 patients with primary hypercholesterolemia. After ≥2 weeks on the National Cholesterol Education Program (NCEP) Step I or a stricter diet and a 4- to 8-week single-blind placebo lead-in, patients with low-density lipoprotein (LDL) cholesterol 130 to 250 mg/dl and triglycerides ≤350 mg/dl were randomized 3:1 to receive ezetimibe 10 mg or placebo orally each morning for 12 weeks. The primary efficacy end point was the percent reduction in direct plasma LDL cholesterol from baseline to end point. A total of 434 men and 458 women (ages 18 to 85 years) received randomized treatment (666 ezetimibe 10 mg, 226 placebo). Demographics and baseline characteristics were similar between treatment groups. Ezetimibe significantly reduced direct LDL cholesterol by a mean of 16.9%, compared with an increase of 0.4% with placebo (p <0.01). Subgroup analysis indicated that response to ezetimibe was generally consistent across all subgroups, regardless of risk-factor status, gender, age, race, or baseline lipid profile. Ezetimibe effects on LDL cholesterol occurred early (2 weeks) and persisted throughout the 12-week treatment period. Compared with placebo, ezetimibe 10 mg also significantly improved calculated LDL cholesterol, apolipoprotein B, total cholesterol, triglycerides, high-density lipoprotein (HDL) cholesterol, and HDL₃ cholesterol (p <0.01). Ezetimibe was well tolerated. There were no differences in laboratory or clinical safety parameters, or gastrointestinal, liver, or muscle side effects from that of placebo. Ezetimibe 10 mg/day is well tolerated, reduces LDL cholesterol approximately 17%, and improves other key lipid parameters.