Patients with homozygous sitosterolemia 10 years of age and older may be eligible for this study. Participants will have a medical history and physical examination and will be randomly assigned to one of two treatment groups. One group, which will include about 80 percent of all study participants, will take 10 mg of Ezetimibe a day, and the second group (20 percent of participants) will take a placebo (an inactive look-a-like pill).