Background: To prevent atherothrombotic events, clopidogrel and aspirin is currently routinely used in treatment of patients undergoing percutaneous coronary intervention (PCI). Despite clopidogrel therapy, patients undergoing PCI are at risk of recurrent coronary events. Therefore, we sought to prospectively evaluate the death and myocardial infarction (MI) of acute coronary syndrome patients and their responsiveness to clopidogrel. Method: We enrolled consecutive 610 patients (pts, 160 males, 65.2짹10.3 years) who received percutaneous coronary intervention (PCI) with acute coronary syndrome (Unstable angina, non-ST elevation MI and ST elevation MI) from Jan. 2006 to Jun. 2008. Endpoint was defined by cardiac death and stent thrombosis (ST) by definitions of the Academic Research Consortium (ARC). Aspirin and clopidogrel responsiveness were evaluated by VerifyNow��� tests (Accumetrics Inc, CA). Clopidogrel low-responsiveness was defined as the less than 20%.inhibition of P2Y12 receptor. Results: Baseline demographic characteristics were similar between normal group (370 pts) and low responsiveness group (240 pts) of clopidogrel. Cardiac death occurred in 7 pts (1.9 %) of normal group and 14 pts (5.8%) in low group (p=0.009). Stent thrombosis occurred in 5 pts of normal group (0.7%, 4 definit and 1 probable) and 10 pts of low group (4.2 %, 7 definite, 2 probable and 1 possible)(p=0.028). The associations between cardiac death and clopidogrel low-responsiveness were evaluated with multivariable logistic regression models adjusted for age and sex. The adjusted Odds ratio for cardiac death was 3.242 (p=0.013, 95%CI ; 1.281-8.205) Conclusions: The low-responsiveness of clopidogrel measured with a point-of-care assay is an independent predictor of cardiac death and stent thrombosis in acute coronary syndrome patients undergoing PCI.