Background and Objectives: VeryfyNow (VN) P2Y12 point-of-care assay (Accumetrics, San Diego, California) has been increasingly utilizing to predict future thromboembolic events by assessing residual platelet reactivity in patients taking clopidogrel. The present study was conducted to investigate whether low responsiveness to clopidogrel assessed by VN P2y12 assay after percutaneous coronary intervention (PCI) may lead to higher occurrence of stent thromobosis (ST) in acute coronary syndrome (ACS). Methods: The study cohort was comprised of 703 ACS patients (465 males, 62.7짹10.7 years) performed VN P2y12 assay following PCI from July, 2006 to March, 2007 at Wonju Christian Hospital and Seoul National University Hospital. Low response to clopidogrel was defined as less than 10% inhibition of baseline PRU or more than 285 IU of PRU, which were lower and higher quartile values. Results: ST was occurred in 21 (2.9%) patients. There was no difference of mean PRU and % inhibition of PRU between ST and no ST groups but somewhat more low responders in subjects with ST without statistical significance (Table). Low responsiveness was not a predictor of ST on the multiple logistic analysis (HR 0.972, CI 0.260-3.639, p=967). Kanplan-Meier curves and the Log-rank test showed no differences between low and normal responders in terms of ST (p=0.286, figure) Conclusion: In ACS setting, post-PCI measurement of VN P2y12 assay does not predict the occurrence future stent thrombosis.