The aim of this study was to elucidate the pharmacokinetics and pharmacodynamics of warfarin enantiomers in relation to cytochrome P450 2C19 (CYP2C19) genotypes.Fourteen subjects, of whom seven were homozygous extensive metabolizers (hmEMs) and seven were poor metabolizers (PMs) for CYP2C19, were enrolled. After a single oral 10 mg dose of racemic warfarin, the plasma concentrations of the warfarin enantiomers and prothrombin time expressed as international normalized ratio (PT-INR) were measured over the course of 120 h.The mean plasma concentrations and elimination half-life of (R)-warfarin of all the subjects were about 2-fold greater than those of (S)-warfarin. Additionally, the area under the plasma concentration–time curve from zero to infinity (AUC0–∞) and the elimination half-life of (R)-warfarin in PMs were significantly greater than those in hmEMs (P = 0·0005 and P = 0·0101 respectively). The S/R ratios of AUC of warfarin enantiomers were 0·51 in hmEMs and 0·37 in PMs (P = 0·0052). Whereas no difference was found in all pharmacokinetic parameters of (S)-warfarin in hmEMs compared with PMs. No significant difference in PT-INR, used as a measure of anticoagulant effect, was found between the hmEMs and PMs.These results show that CYP2C19 activity is important in the pharmacokinetics of (R)-warfarin. However, when warfarin is administered as a racemate, this difference is not translated into any significant effect in the pharmacodynamics of warfarin.