Null genotype of glutathione S-transferase class me (GSTM1(-)) and the plasma level of malondialdehyde (MDA) were determined in normal subjects, epileptics with good seizure control and epileptics with intractable seizure. The frequency of GSTM1(-) of the epileptics with intractable seizure was significantly higher than that of the group with good seizure control and that of the normal subjects. The average plasma level of MDA of the patients with GSTM1(-) genotype was higher than that of the subjects with GSTM1(+) in either the normal or epileptic group. Thus, the GSTM1(-) genotype may be one of the genetic factors involved in the response to anticonvulsant therapy.