Regional cerebral blood flow (rCBF) was measured by single photon emission tomography, using Xenon-133 inhalation during migraine attacks. The method yields images of three brain slices, depicting rCBF with a spatial resolution of 1.7 cm. Twelve patients with common migraine showed a normal cerebral blood flow pattern during an attack. Of 11 patients with classical migraine, 8 displayed a unilateral region of hypoperfusion, while 3 had a normal flow pattern. The focal decrease of blood flow average 17 per cent as compared with the symmetrical contralateral region. The localization of the blood flow reduction corresponded to the tomographic representation of the lateral aspect of the cortex in 8 patients, while in one patient the mesial aspect also appeared to be involved. The low-flow regions were always on the side opposite to the focal neurological symptoms. The hypoperfusion remained through the headache period for four to six hours. Regions of relative or absolute hyperaemia were not observed during headache of either form of migraine. Between attacks, rCBF was normal except in one patient who had a minor low-flow region in the insula. The cortical distribution of the vasomotor changes in classical migraine appeared unrelated to the distribution of major cerebral arteries and may represent a manifestation of disturbed cerebral metabolism, possibly the spreading depression of Leão. Since classical migraine in most cases was associated with intracranial blood flow changes while common migraine was not, the study was taken to support that these two forms of migraine may differ, not only on a clinical basis, but also on pathophysiological grounds.