Clobazam, 20 or 30 mg/day was given for 10 days around menstruation in successive menstrual cycles to 13 women who had responded favourably to this drug in an earlier short-term placebo controlled cross-over study. Three patients have been successfully treated, with complete freedom from seizures around menstruation, for 3-3 1/2 years and two others responded favourably until pregnancy made treatment inappropriate. A further four patients did well during a shorter period (6-13 months) of follow-up. An increase in seizures between periods of clobazam therapy was observed in three patients, and led to the withdrawal of this drug in two of them. However, tolerance to the antiepileptic effect of clobazam was not observed in any patient, even though nine were treated for 1 year or more. In only once case was it necessary to discontinue treatment because of sedative side effects.