We investigated the influence of genetic polymorphisms of CYP2C9 and VKORC1 genotypes on the pharmacokinetics and pharmacodynamics of warfarin and established an equation for predicting the maintenance dose of warfarin in the Thai population using genetic and non-genetic factors. The CYP2C9*2, CYP2C9*3, VKORC1 C1173T and VKORC1 G-1639A genotypes were detected by realtime PCR using fluorogenic hybridization probes. The associations between genetic and demographic factors with respect to warfarin dosage were analyzed. CYP2C9 polymorphisms affect warfarin metabolism as shown by a significant difference in warfarin clearance, whereas VKORC1 genotypes cause a significant difference in warfarin sensitivity index (INR:Cp). The mean weekly warfarin dose was significantly different among different VKORC1 and CYP2C9 genotypes. Patients with the VKORC1 BB haplotype and CYP2C9*1/*1 required about twice the warfarin dose compared to those with the VKORC1 AA haplotype and CYP2C9*1/*1. Using stepwise multiple linear regression, clinical factors (age and weight) and genetic factors (CYP2C9 and VKORC1) could explain about 53.8% of the variance of the warfarin maintenance dose. CYP2C9 and VKORC1 genotypes played an important role in the inter-individual variation in warfarin maintenance dose in a Thai population.