SCH-58235 is a member of a new class of therapeutic agents, specific cholesterol absorption inhibitors, being developed to treat hypercholesterolemia. Preclinical animal studies and clinical studies in humans have demonstrated significant reductions in serum cholesterol levels with SCH-58235 treatment. In the current study, the efficacy and safety of SCH-58235 will be studied in patients with homozygous sitosterolemia, an inherited condition in which a wide range of plant sterols, including sitosterol, are excessively absorbed and inadequately excreted, leading to dramatic increases in tissue deposition. SCH-58235 may lower sitosterol levels by directly inhibiting sitosterol absorption (since plant sterols and cholesterol are structurally similar sterols) and/or by inhibiting cholesterol absorption and indirectly by increasing hepatic uptake of plasma LDL with increased cholesterol and sitosterol excretion. These mechanisms of action could lead to reductions in both plant sterol and LDL cholesterol levels in these patients, thereby providing important clinical benefits. The current study will determine if SCH-58235 will be safe and well tolerated and provide significant reductions in plant sterols (plasma sitosterol and campesterol) and low-density lipoprotein-cholesterol (LDL-C) concentrations in patients with homozygous sitosterolemia.