This study was undertaken to determine whether migraine attacks exhibits circadian, menstrual, or seasonal variations in frequency and, thus, to characterize more precisely this relapsing, remittent, pleomorphic disease. An analysis of 3582 well-documented migraine attacks in 1698 adults was undertaken. The demographics of the study population accurately represented the known epidemiology of the disease. Migraine attacks started more frequently between 4 AM and 9 AM and within the first few days after onset of menses; this migraine periodicity is strongest amongst women not using oral contraceptives. Seasonal periodicity, if any, is clearly weaker than circadian or menstrual. These chronobiological features may assist in the differential diagnosis of migraine from premenstrual headache and fibromyalgia.