Cerebral blood flow has seldom been measured during attacks of migraine and cluster headache. The literature is reviewed and five cases studied in our laboratory are described. The results of these studies confirm Wolff's hypothesis that cerebral blood flow is decreased during auras and increased during headaches. However, the distribution in time and space of the blood flow changes do not always correlate with the clinical features of the attack. Autoregulation of cerebral blood vessels may be impaired in aura and headache, and this may be a factor in intensifying and prolonging attacks.