The incidence of migraine varies over the course of the menstrual cycle. In the general population, approximately 60% of women with migraine report an increased frequency of headache during menses. The estrogen withdrawal that occurs just prior to the onset of menses and that leads to loss of serotonergic tone is thought to be the trigger for headaches that arise at this time of the menstrual cycle. The ability of triptans, specific serotonin receptor agonists, to prevent menstrual migraine is consistent with this hypothesis. Moreover, compared with headaches that occur during other times in the cycle, menstrual migraines are more severe in most women and may be of longer duration, as well as more resistant to treatment in a subset of women.