The aim of our study is to Estimate the reduction of LDL by ezetimibe in hypercholesterolemic patients on simvastatin.Investigate the effect of LDL lowering by ezetimibe on platelet activity and LDL tendency to peroxidation in hypercholesterolemic patients on simvastatin therapy

The hypothesis is that:

1. LDL lowering by ezetimibe on-top of simvastatin in patients on fixed dose of simvastatin can reduce platelet aggregation, due to the potential decreasing of cholesterol content in the platelet membranes.
2. LDL lowering by ezetimibe can lower LDL tendency to peroxidation.

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.