All women with migraine are susceptible to the effects of hormonal changes. For a minority with menstrual migraine, fluctuating hormones of the normal ovarian cycle are a specific trigger, particularly during perimenopause. The author proposes that the term menstrual migraine should be restricted to migraine attacks occurring on day 1 +/- 2 days of the menstrual cycle with freedom from migraine during the rest of the cycle. This definition is compatible with the mechanism of estrogen withdrawal. Other mechanisms such as prostaglandin release also may be important for some women. The changing hormonal environment at various stages of life provides further evidence of the role of estrogen in migraine. Treatments that stabilize hormone levels in the form of estrogen supplementation for menstrual migraine, elimination of the pill-free week, and adequate, stable levels of estrogen for HRT, all are associated with an improvement in migraine. The control of the menstrual cycle, however, is extremely complex, and until further studies are undertaken using strict criteria, the mechanism of migraine triggered by hormonal events remains uncertain.