Inclusion Criteria:

1. Hypercholesterolemic patients on stable simvastatin dose for at least one month.
2. Age ≥18 years on stable AHA step 1 diet.
3. Patients without CHD or with one risk factors ; LDL > 130 mg/dL and for Patients with CHD or CHD risk equivalent(clinical manifestations of non-coronary forms of atherosclerotic disease) or with 2 risk f actors (cigarette smoking, hypertension (BP ≥140/90 mm Hg or on antihypertensive medication), low HDL cholesterol (<40 mg/dL), family history of premature CHD;LDL>100 mg/dL
4. Patients `on at least simvastatin treatment of 20 mg per day.
5. CPK, ALT and AST < 1.5 X upper limit of normal at baseline.

Exclusion Criteria:

1. Women currently receiving cyclical hormones.
2. Treatment with psyllium, other fiber based laxatives, phytosterol margarines, or other OTCs that affect serum lipids, unless treated with a stable regimen for > 6 weeks and the patient agrees to continue this regimen for the duration of the trial.
3. Oral corticosteroids unless used as replacement therapy for pituitary/adrenal disease and a stable regimen for at least 6 weeks.
4. Lipid lowering agents including fish oils and QUESTRAN taken within 6 weeks.
5. Active coronary heart disease: unstable angina, acute myocardial infarction, CABG or PTCA within the last 3 months.
6. Women with childbearing potential unless on safe contraception.
7. Psychiatric disease with defect in judgement.
8. Severe renal or hepatic disease.
9. Uncontrolled hypo- or hyperthyroidism.
10. Contraindication for ezetimibe or simvastatin treatment. The patients will continue on their treatment with simvastatin for 6 weeks, and then the patients will be treated by the same dose of simvastatin combined with ezetimibe 10 mg/day for other 6 weeks.