The aim of this study was to prospectively assess the safety and efficacy of lovastatin in the treatment of cholesteryl ester storage disease in siblings who were ages 11.6 and 5 y at the beginning of treatment. Mean total and LDL cholesterol in the male proband, 7.40 and 5.68 mmol/L, respectively, on diet alone, fell 30% to 5.2 (p < or = 0.001) and 31% to 3.9 mmol/L (p < or = 0.001) on lovastatin 40 mg/d over 3.3 y, with simultaneous resolution of hepatosplenomegaly. In his sister, on lovastatin 20 mg/d for 1.5 y, total and LDL cholesterol fell, but not significantly; her hepatosplenomegaly was also reduced on treatment. Lovastatin was well tolerated without overt side effects or complications and without adverse changes in liver function tests or creatine phosphokinase. Normal and expected accretion of height and weight occurred during the treatment period for both children. Lovastatin appears to be a safe and effective treatment for pediatric cholesteryl ester storage disease.