Forty women of childbearing age with refractory epilepsy were asked to record their seizures, the first and last days of their menstrual periods, and symptoms of premenstrual tension for 3 consecutive months. By defining catamenial epilepsy as the occurrence of at least 75% of seizures each month in the 10-day time frame, which included the 4 days preceding menstruation and the 6 days after its onset, only 5 women (12.5%) were identified who fulfilled the criterion. Nevertheless, after the study was completed, 31 (78%) of these patients claimed that most of their seizures occurred near the time of and were exacerbated by menstruation. The patients with catamenial epilepsy reported no more symptoms of premenstrual tension than did the rest of the group. Clustering outside the menstrual cycle was noted in 4 other patients. Catamenial epilepsy is an uncommon condition. Patient claims about frequency of seizures in relation to menstruation are not always accurate. A standard definition should be adopted because the diagnosis has implications for management.