Regional cerebral blood flow (rCBF) was measured in seven patients during classic migraine attacks. Single photon emission computed tomography was performed soon after hospital admission, and three to eight hours, 20 to 24 hours, and one week later after the onset of symptoms. Initially reduced rCBF persisting up to three hours was observed in the hemisphere appropriate to the focal neurologic deficit; hyperperfusion was noted later in the same region in these patients. At 24 hours rCBF was normal in four patients, hyperemia persisted in two patients, and one patient was not restudied. The area of interest demonstrated a mean decrease of 19% +/- 7% in side-to-side asymmetry when compared with the contralateral region. In three to eight hours this reversed to a mean increase of 19% +/- 4% (delayed hyperemia). No asymmetries were observed after one week. The late hyperemic asymmetry often persisted beyond the duration of the clinical headache. It is postulated that this tardive regional hyperperfusion is a consequence of previous focal arteriolar vasoconstriction. This vascular sequence of events further delineates the diagnostic merit of studying rCBF by noninvasive single photon emission computed tomography in migraine.