This study will test the safety and effectiveness of SCH-58235 (Ezetimibe) in lowering sitosterol, plant sterol and cholesterol blood levels in patients with homozygous sitosterolemia when added to the patients' current treatment regimen. Homozygous sitosterolemia is an inherited disorder of sterol metabolism in which an excess of many plant sterols, including sitosterol, is absorbed and not enough excreted. (Sterols are substances used to form hormones, vitamins and membranes found in animal and plant lipids.). Patients can develop atherosclerosis with coronary heart disease as early as childhood, as well as other problems including arthritis, arthralgia, and tendon xanthomas (lipid deposits). Current sitosterolemia treatments may include a low sterol diet, medications, intestinal surgery, or a combination of these. Ezetimibe is a member of a new class of drugs called "specific cholesterol absorption inhibitors" that may lower cholesterol, sitosterol and other plant sterol blood levels.