Abstract: AIM: To assess the phenotype and genotype pattern of S-mephenytoin 4'-hydroxylation in Chinese population. METHODS: The phenotypes of ninety healthy subjects were analyzed with S/R mephenytoin ratio in urine after an oral dose of 100 mg racemic mephenytoin by chiral GC-FID method. The genotypes of twenty-six among the 90 subjects were analyzed with identifying the wild-type(wt) gene and two mutations, CYP2C19m1 and CYP2C19m2 by PCR method. RESULTS: Of the 90 subjects eleven were identified as poor metabolizers with the S/R ratio of 0.95. Among the 26 genotyped subjects six were homozygous for wild-type (wt/wt); nine were homozygous for CYP2C19m₁(m₁/m₁); seven were heterozygous for the CYP2C19m1(wt/m₁); three were heterozygous for the CYP2C19m₂(wt/m₂); one was the heterozygous for the two defects (m₁/m₂). CONCLUSION: The result of CYP2C19 genotype analysis was in agreement with that of phenotype analysis. The frequency of PM by phenotype analysis was 12.2%.