Purpose: To investigate seizure frequency in relation to menstrual cycles and seizure exacerbations with respect to various clinical factors in women with epilepsy. Methods: The authors prospectively evaluated premenopausal women with epilepsy aged 15–44. Catamenial epilepsy was defined as seizure frequency during the perimenstrual (C1), periovulatory (C2) or non-menstrual phase (C3) at least twice that during other phases. Results: In total 255 menstrual cycles, 231 ovulatory and 24 anovulatory cycles were registered in 79 patients (29.7 ± 7.8 years old). Average seizure frequency was 0.13 ± 0.29/day during the menstrual phase, 0.14 ± 0.24 during the follicular, 0.18 ± 0.61 in the ovulatory, and 0.14 ± 0.33 during the luteal phases. Catamenial seizure exacerbation was observed in 37/79 (46.8%) patients and 108/255 (42.4%) cycles, more frequently during anovulatory (17/24, 70.8%) than ovulatory (91/231, 39.4%) cycles (p = 0.003). During ovulatory cycles, seizure exacerbation was primarily C1 (42.9%) or C2 (45.1%) pattern. Myoclonic seizures were more frequently associated with the C1 pattern. Conclusions: Overall, 46.8% of women had catamenial epilepsy. Seizure frequencies were greater during menstrual and ovulatory phases for ovulatory cycles, and during non-menstrual phases for anovulatory cycles.