Abstract: Objective To evaluate the effect of CYP2C19 polymorphisms on the phenytoin in the treatment of epilepsy.Methods A total of 150 epileptic patients treated with oral phenytoin in the Department of Neurology of our hospital between January 2009 and June 2010 were included in the study.Genotyping of CYP2C19 polymorphisms was examined by PCR-restriction fragment length polymorphism (RFLP).High performance liquid chromatography (HPLC) was performed to detect the plasma concentrations of phenytoin in the patients.The relationship between CYP2C19 polymorphisms and the patient response to phenytoin was evaluated.Results Of the 150 patients,those who responsed to the phenytoin treatment were 44 cases of CYP2C19*2/*2,7 of CYP2C19*3/*3 and 1 of CYP2C19*2/*3,while those without response were 1 case of CYP2C19*2/*2,1 of CYP2C19*3/*3,and 96 of CYP2C19*2/*3.There was a statistical difference in the intergroup comparison (P<0.05).In patients who responded to phenytoin treatment,there was no significant difference in plasma concentration of phenytoin among genotypes (P>0.05).Conclusion CYP2C19 polymorphism is associated with patient response to phenytoin.