A direct chiral-phase high-performance liquid chromatographic method for measuring the ratio of S-warfarin/R-warfarin in patient plasma is described. Plasma samples are first extracted using solid-phase C18 extraction columns, and the concentrated extracts analyzed using an (R,R) Whelk-O 1 column with a mobile phase of 0.5% glacial acetic acid in acetonitrile. The resulting chromatography provides baseline resolution of the warfarin enantiomers and internal standard (racemic ethylwarfarin), and is free from interference from other plasma components. Calibration curves were linear (mean r2 of 0.999 for both enantiomers) over the concentration range 0.25-1.5 microg/ml. The intra-day and inter-day coefficients of variation for analysis of plasma spiked with 0.33 microg/ml S-warfarin and 0.67 microg/ml R-warfarin (S/R=0.5:1) was less than 7% for each enantiomer, with an accuracy of more than 93%. Plasma extracts from thirty-one patients homozygous for wild-type CYP2C9*1 provided an S/R ratio of 0.51+/-0.15. Two warfarin patients homozygous for the mutant CYP2C9*2 and CYP2C9*3 alleles exhibited elevated S/R ratios relative to the mean for individuals homozygous for the wild-type CYP2C9*1 allele. This method is suitable for population studies aimed at establishing the effect of polymorphic expression of CYP2C9 alleles on S-warfarin elimination in humans.