Background: Clopidogrel with aspirin is widely administered to prevent stent thrombosis. But aspirin resistance and interindividual variability in response to clopidogrel make their prognosis worse especially in high platelet activity, which implying more effective antiplatelet strategy. Recent studies showed that triple antiplatelet therapy(cilostazol+clopidogrel+aspirin) resulted in a significantly lower restenosis rate after coronary stenting than conventional therapy(clopidogrel+aspirin) and platelet P-selectin supression might have a some role for reduced restenosis rate. However, antiplatelet effects of cilostazol when combined with clopidogrel and aspirin was not known. We assessed antiplatelet properties of triple therapy. Methods: Low dose cilostazol(50mg/BID) was given to 47 patients who already had been taking clopidogrel and aspirin for more than 1 month after coronary stenting. The expression of P-selectin and activated GPIIb/IIIa were measured by whole blood flowcytometric analysis as a marker of platelet activation before cilostazol treatment and 2 weeks after cilostazol treatment. Results: P-selectin was significantly decreased after 2 weeks of cilostazol treatment( 3.2짹2.4% to 2.0짹1.9%, p=0.03), especially in patients of relatively high pretreatment P-selectin activity, but activated GPIIb/IIIa was not significantly changed (13.5% to 17.6%, p>0.05). Underlying disease, concomitant medication including statin, and CRP was not related to platelet activation. Conclusion: Additional cilostazol treatment to conventional antiplatelet therapy(clopidogrel+aspirin) after coronary stenting provides more effective suppression of platelet p-selectin expression especially in patients with relatively high pre-treatment platelet activity, suggesting cilostazol may prevent adverse clinical outcomes in poor responder to clodpidogrel. Thus, this combination regimen deserves further evaluation in clinical and prognostic value in coronary intervention.