 The aims of this study is to evaluate the effects of platelet responsiveness to clopidogrel or cilostazol in CKD patients undergoing hemodialysis. The differences in platelet activation markers are also evaluated before and after clopidogrel or cilostazol administration. The investigators will perform a prospective, randomized study to compare the degree of platelet inhibition and platelet activation markers by adjunctive cilostazol 100 mg twice daily compared to clopidogrel 75 or 150 mgday in CKD patients undergoing hemodialysis. 