This investigation assessed the frequency of catamenial epilepsy in 87 women who charted seizures and menses during three cycles. Catamenial epilepsy designation was made if two of three cycles showed at least one of three previously defined catamenial patterns. Among ovulatory cycles, average daily seizure frequency was significantly greater during the perimenstrual and preovulatory phases. Among anovulatory cycles, average daily seizure frequency was substantially less during the midfollicular phase than during the remainder of the cycle. Overall, 39.1% of the women had catamenial epilepsy.