Background: Clopidogrel resistance has been associated with an adverse long-term outcome in patients with atherosclerotic coronary artery disease, but it's not clear that the adjuvant antiplatelet drug in clopidogrel resistant patients is necessary. Purpose: The aim of this study was the evaluation of long-term outcome in ACS with periprocedual clopidogrel resistance and verifying effect of GpIIb/IIIa antagonist in clopidogrel resistant patients. Methods and Results: From June 2006 to May 2008, consecutive, 152patients with STEMI or NSTEMI were enrolled who underwent successful PCI. 300mg clopidogrel was loaded on the 1st day, Clopidogrel 600mg was given before stenting and a daily 75mg dose was administered on the following days. Clopidogrel resistance were assayed at the next day after PCI. The percent inhibition of < 20% indicates the absence of clopidogrel induced platelet dysfunction and was defined as clopidogrel resistance by VerifyNow System. We investigated 9months major adverse cardiologic events (MACE: cardiac death, MI, stent thrombosis, target vessel revascularization) respectively. Among the 152 studied patient, 69 patients (45.4%) were resistant to clopidogrel. There was no significant difference in baseline cardiologic risk factors between clopidogrel resistant patients (Group I) and clopidogrel sensitive patients (Group II). The incidence of MACE within the 9months period was significantly higher in Group I (17.4% vs 8.4%, p=0.031). Also subgroup analyses were performed in Group I (subgroup I: GpIIb/IIIa antagonist infusion, subgroupII: GpIIb/IIIa antagonist not infusion) and Group II (subgroupIII: GpIIb/IIIa antagonist infusion, subgroupIV: GpIIb/IIIa antagonist not infusion). There was no significant difference in subgroupIII and subgroupIV, but the MACE was lower in subgroup I than subgroup II(23.9% vs 4.3% p=0.043). Conclusions: Our study demonstrates that identification of immediate periprocedual clopidogrel resistance is important to the long-term prognosis in ACS patients, and suggest that the adjuvant antiplatelet drug intervention such as GpIIb/IIIa antagonist may be necessary in clopidogrel resistant patients.