Introduction: High platelet reactivity despite clopidogrel therapy is a risk factor for ischemic complication in patients undergoing percutaneous coronary intervention. Polymorphisms of cytochrome (CYP) 2C19, 3A4, 3A5 have been reported as important factors affecting clopidogrel responsiveness, but its role in asian population is still unclear. Methods: From October 2006 to May 2007, 410 patients who underwent percutaneous coronary intervention with drug-eluting stents were randomly assigned to dual antiplatelet regimen (aspirin plus clopidogrel) or triple antiplatelet regimen (aspirin plus clopidogrel plus cilostazol). Clopidogrel resistance and genetic analysis were done in 383 patients. Clopidogrel resistance was defined as % inhibition of less than 20% by VeryfyNow P2Y12 assay. Results: Clopidogrel resistance was found in 111 patients (29.0%). CYP2C19*3 polymorphism and triple antiplatelet regimen showed significant relationship with clopidogrel resistance. The patients were subdivided into two groups according to their antiplatelet regimen to exclude possible confounding effect of cilostazol. CYP2C*19*3 polymorphism demonstrated significant relationship regardless of antiplatelet regimen (table 1). Multiple logistic regression analysis demonstrated that CYP2C19*3 polymorphism (Dominant: HR=2.51, 95%CI; 1.42-4.45, p=0.002, Codominant: HR=2.46, 95%CI; 1.43-4.3, p=0.001) was associated with clopidogrel resistance.