This is a multicenter, randomized, double-blind, placebo-controlled study lasting 12 weeks. Patients with a diagnosis of homozygous sitosterolemia who have an elevated plasma sitosterol level on their current regimen will be studied. Patients will be eligible to enroll in the study whether their current regimen is medication(s), surgical (prior ileal bypass), apheresis, a combination of treatments, or no active regimen. The primary objective of this study is to evaluate the efficacy and safety of 10 mg/day of SCH-58235 in patients with homozygous sitosterolemia who continue to have an elevated plasma sitosterol level.