Background— A multicenter, randomized, double-blind, placebo-controlled study was conducted to evaluate LDL cholesterol–lowering efficacy, overall safety, and tolerability and the influence on growth and pubertal development of simvastatin in a large cohort of boys and girls with heterozygous familial hypercholesterolemia (heFH). Methods and Results— A total of 173 heFH children (98 boys and 75 girls) were included in this study. After a 4-week diet/placebo run-in period, children with heFH were randomized to either simvastatin or placebo in a ratio of 3:2. Simvastatin was started at 10 mg/d and titrated at 8-week intervals to 20 and then 40 mg/d. During a 24-week extension period, the patients continued to receive simvastatin (40 mg) or placebo according to their assignment. After 48 weeks of simvastatin therapy, there were significant reductions of LDL cholesterol (−41%), total cholesterol (−31%), apolipoprotein B (−34%), VLDL cholesterol (−21%), and triglyceride (−9%) levels. HDL cholesterol and apolipoprotein A-I levels were increased by 3.3% and 10.4%, respectively (not significant). No safety issues became evident. Except for small decreases in dehydroepiandrosterone sulfate compared with placebo, there were no significant changes from baseline in adrenal, gonadal, and pituitary hormones in either treatment group. Conclusions— Simvastatin significantly reduced LDL cholesterol, total cholesterol, triglyceride, VLDL cholesterol, and apolipoprotein B levels and was well tolerated in children with heFH. There was no evidence of any adverse effect of simvastatin on growth and pubertal development. Therefore, simvastatin at doses up to 40 mg is a well-tolerated and effective therapy for heFH children.