Seizures are influenced by the physiologic variation in sex hormone secretion during the menstrual cycle and throughout the reproductive life of women with epilepsy. Catamenial epilepsy is defined as the occurrence of seizures around menses or an increase in seizures in relationto the menstrual cycle. The incidence of catamenial epilepsy varies from 10% to 78%, largely because of methodological differences among studies. Variation in concentracion of antiepileptic drugs across the menstrual cycle may also contribute to increased seizure susceptibility. It is important for the physician to work closely with the patient to determine whether her seizures are indeed catamenial and to design an appropriate treatment plan.