Background. Warfarin use is complicated by an erratic dose response. Interpatient variability associated with warfarin therapy may be partly attributable to polymorphisms of the cytochrome P450 (CYP) complex. The purpose of this study was to ascertain the frequency and influence of CYP polymorphisms on warfarin dosing in our patient population. Methods. Patients receiving warfarin therapy were recruited from the inpatient divisions of our hospital. Genotyping for known polymorphic alleles of the CYP subfamilies CYP2C9 (CYP2C9*1, CYP2C9*2, and CYP2C9*3) nd CYP2A6 (CYP2A6*1, CYP2A6*2) with the use of standard methods of polymerase chain reaction amplification was performed. An unpaired t test was used to statistically compare genotypes. Results. Genotype frequency in 38 patients is as follows: CYP2C9*1 /CYP2C9*1,71%; CYP2C9*1 /CYP2C9*2, 21%; CYP2C9*2/CYP2C9*2, 3%; CYP2C9*1/CYP2C9*3, 5 %; CYP2A6*I/CYP2A6*I, 95%; CYP2A6*I/CYP2A6*2, 5%. Compared to a wild-type genotype, the presence of the CYP2C9*2, CYP2C9*3, or CYP2A6*2 allele was associated with a significant reduction in weekly warfarin dose (mean weekly warfarin dose [± SE] for wild-type genotype was 0.397 ± O. 024 mg/kg/wk vs 0.307 ± 0.03 mg/kg/wk for carriers of CYP2C9*2, CYP2C9*3, or CYP2A6*2 polymorphism; P = .03). Conclusions. Polymorphisms that impair warfarin metabolism are common, occurring in 34 % of patients, and are associated with increased warfarin sensitivity. The use of genotypic information to prescribe more accurate doses of warfarin may increase the safety and efficacy of this medication.