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Background: Warfarin dosage requirements show considerable inter-individual variability. There are some reports of warfarin dose regimens correlating with single nucleotide polymorphisms (SNP) for CYP2C9, VKORC1 and other genes in adults. In children, however, reports are scarcer than in adults and the number of genes examined is more limited. We explored the effects of genetic variation on warfarin dose requirement in Japanese pediatric patients. Methods: A total of 45 patients who were prescribed warfarin at the Yokohama City University Hospital were included in this study. The influence of genetic polymorphisms on stable warfarin dosage requirement was investigated by genotyping SNPs of the VKORC1, CYP2C9, CYP4F2, and GGCX genes (rs9923231, rs1057910, rs2108622, and rs699664, respectively) in each patient. Results: Patients with the TT genotype in rs9923231 in VKORC1 required significantly lower maintenance dosages than those with the TC genotype ( p = 0.001). Multiple regression analysis showed that, while VKORC1 status and patient height account for 78.2 % of the variability in maintenance warfarin dosage, genetic polymorphisms in VKORC1 account for 27 %, although polymorphisms in CYP4F2 and GGCX had no effect on dosage and the effect of CYP2C9 could not be evaluated. Conclusions: Polymorphisms in VKORC1 partially affected daily warfarin dosage requirements. VKORC1 genotype and height are the primary determinants influencing warfarin dosage in Japanese pediatric patients. Further studies with larger sample sizes are needed to confirm our results. [ABSTRACT FROM AUTHOR]