This open trial assessed the effects of adjunctive progesterone therapy on seizure frequency in 25 women with catamenial exacerbation of complex partial (CPS) and secondary generalized motor (SGMS) seizures. Progesterone was well tolerated by 23 of the 25 women and had readily reversible dose-related side effects of asthenia and emotional depression in two. Eighteen women (72%) experienced a decline in seizure frequency during a 3-month treatment period compared with the 3 months prior to therapy (p < 0.01). Average daily CPS frequency declined by 54% (p < 0.01), SGMS by 58% (p < 0.02).