The incidence of the S-mephenytoin polymorphism was compared in two Chinese ethnic groups, Han (n=101) and Bai (n=202) by phenotype and genotype analysis. The frequency of poor metabolizers (PMs) in Han vs. Bai subjects was 19.8% vs. 13.4%. Han subjects had a higher frequency of the mutant CYP2C19m1 allele (0.366 vs. 0.257, P＜.01) and a lower frequency of the wild-type allele (0.559 vs. 0.688, P＜.01) than Bai subjects, which is consistent with the difference in the frequencies of PMs between the two ethnic groups. This results in a lower percentage of homozygous wild-type extensive metabolizers of mephenytoin (EMs) in Han subjects than in Bai subjects (40% vs. 59%, P=.005). Therefore, Han subjects may be more susceptible than Bai subjects to the drugs metabolized by the CYP2C19 enzyme. Ratios of urinary S/R-mephenytoin in homozygous EMs were lower than those of heterozygous EMs for both Han and Bai subjects, which shows a gene-dosage effect. Genotype analysis identified all but one PM as homozygous or heterozygous for the two known mutant CYP2C19m1 and/or CYP2C19m2 alleles. A single Bai PM outlier was shown to be heterozygous for CYP2C19m1 and a new mutant CYP2C19 allele containing a single amino acid change of Arg4333Trp433. A genotyping test demonstrated that only this one individual carried this rare allele (frequency of 0.0025 in Bai subjects).