Background: CYP2C19 loss-of-function genotype (*2 and/or *3 alleles) is related to low responsiveness to clopidogrel, which is a risk factor for ischemic cardiac events. The contribution of these genotypes to platelet reactivity in Japanese patients in a steady state receiving dual antiplatelet therapy after coronary stenting was evaluated. Methods and Results: A total of 155 Japanese patients were classified according to their CYP2C19 loss-of-function genotype. Platelet reactivity was assayed by plasma levels of soluble P-selectin and platelet-derived microparticles, light transmittance aggregometry induced by ADP (ADP-LTA), shear stress-induced platelet aggregometry, vasodilator-stimulated phosphoprotein phosphorylation (VASP) index and the VerifyNow-P2Y12 assay. Linear and logistic regression models were used to assess the associations between CYP2C19 loss-of-function genotype and high on-treatment platelet reactivity. In total, 62 patients (40.0%) were extensive metabolizers (EMs), 70 (45.2%) were intermediate metabolizers (IMs) and 23 (14.8%) were poor metabolizers (PMs). ADP-specific assays (ADP-LTA, the VASP index and VerifyNow-P2Y12) differed according to CYP2C19 genotype, with a significant gene-dose effect (PMs>IMs>EMs). CYP2C19 loss-of-function carrier status was associated with more frequent high platelet reactivity. CYP2C19 loss-of-function genotype alone could explain 12.2%, 14.3%, and 14.7% of the variability in the ADP-LTA, VASP and VerifyNow-P2Y12 assays, respectively. Conclusions: CYP2C19 loss-of-function genotype is associated with more frequent high platelet reactivity, as assessed by ADP-specific platelet function tests, in Japanese patients.  (Circ J 2013; 77: 1436–1444)