Introduction: Carriage of CYP2C19 *2 and *3 allele is associated with diminished platelet response to clopidogrel and higher rate of clinical risk. The purpose of this study was to compare cut off levels of light transmittanace aggregometry (LTA) and VerifyNow assay for the prediction of CYP2C19*2 and *3 allele in Korean patients undergoing percutaneous coronary intervention (PCI). Methods: We enrolled 100 coronary artery disease (CAD) patients undergoing PCI with clopidogrel medication. Platelet reactivity was assessed with LTA and the VerifyNow P2Y12 assay and CYP2C19 genotype was analyzed by polymerase chain reaction. Results: the CYP2C19*2 and *3 allele constituted 10 (10%) of 100 CAD patients. Platelet inhibition measured by LTA and VerifyNow was diminished in CYP2C19*1 allele compared with CYP2C19*2 and *3 allele (LTA: 32.6짹17.8 vs 53.9짹16.2 p<0.01; PRU: 221.7짹10.5 vs 300짹43.2, p<0.01). Receiver-operating characteristic (ROC) curve analysis showed that LTA and VerifyNow assays were significant predictors of carrier state (AUC 0.76 95% CI 0.66-0.84 p<0.005 and AUC 0.66 95% CI 0.588-0.757 p=0.098). The cut off values of LTA and VerifyNow were 10µM ADP-induced maximal aggregation>42% by LTA and PRU���272 for the identification of carriers (Fig. 1a & 1b). Conclusions: our results suggested that the cut-off levels of LTA and VerifyNow to discriminate carriers of CYP2C19*2 and *3 from non-carriers are potentially useful to provide optimal clopidogrel therapy in patients with coronary stent implantation.