Background: It has been well established that pre-procedural platelet inhibition influences clinical outcomes after percutaneous coronary intervention (PCI). Recent data have shown that the CYP2C19*2 loss-of-function allele is associated with high post-treatment platelet reactivity after clopidogrel therapy in Caucasian patients with acute coronary syndrome. However there is no large scaled, randomized trial to asses the prevalence of CYP2C19 polymorphism and impact of the CYP2C19 polymorphism on clinical outcome in Asian population with coronary artery disease. Methods: We enrolled prospectively patients undergoing PCI or with documented coronary artery disease (n = 618) to define the prevalence of CYP 2C19 polymorphism from December, 2007 to April, 2009. All consecutive patients were assessed CYP2C19 genotypes and two CYP 2C19 polymorphism, CYP 2C19*2 and CYP 2C19*3, were investigated. The CYP2C19 genotype was analyzed by the SNaPshot method. Results: The number of patients with wild-type allele was 233 (37.7%), one mutant allele was 291 (47.1%), and two mutant allele was 94 (15.2%). The number of patients exhibiting the CYP2C19*1/*1, *1/*2, *1/*3, *2/*2, *2/*3 and *3/*3 genotypes were 232 (37.5%), 220 (35.6%), 69 (11.2%), 60 (9.7%), 34 (5.5%), and 3 (0.5%), respectively. The number of patients with CYP2C19*3, which is very rare in Caucasian population, was 106 (17.5%). Conclusion: The prevalence of patients with CYP 2C19 polymorphism was high compared to that of Caucasina population, moreover, the number of patients exhibiting CYP2C19*3 were high. Therefore impact the CYP2C19 polymorphism on clinical outcome in Asian patients with CAD is needed to be evaluated to reduce adverse outcome after PCI.