Women with epilepsy have been encouraged to consider marriage and child bearing in recent years. Antiepileptic drugs are mostly used such as phenytoin, but its effect on fetus health and its long term effects on DNA have not been enough clear yet. The decision to continue ör initiate pharmacotherapy for epilepsy during pregnancy becomes complicated. Therefore it was aimed to determine the potential toxic effects of long term phenytoin monotherapy on the peripheral blood lymphocytes of female patients with epilepsy using the comet assay. Twenty-three female patients on a long-term treatment of phenytoin monotherapy for 2-6 years were studied. The epileptic female patients who had normal menstrual cycles, and who were in, otherwise, good health were accepted. They were also nonsmokers. Control group consisted of 23 healthy, nonsmoker female patients, who had normal menstrual cycles and did not use any long-term drugs. The blood samples were taken from the control and patient groups within 20th and 27* days following the beginning of their menstrual bleeding. As a result, the statistical comparison of the comet scores of two groups demonstrated that there is a significant difference in number of damaged cells. Damaged (limited and extensive migrated) cells in the epileptic women who were taking phenytoin were higher than the control group (p<0,0001).