There is still some controversy about alterations in velocity of blood flow and in cerebral vasomotor reactivity of intracranial arteries in migraineurs during the interictal phase. By means of simultaneous bilateral transcranial Doppler ultrasonography we, therefore, assessed intracranial blood fIow velocities and cerebrovascular reactivity to carbon dioxide of all three basal brain arteries in 20 migraineurs during the interictal phase and 30 nonheadache-prone control subjects. Mean blood flow velocities were higher in migraineurs than in controls in all three arteries on both sides, with a significant difference (P<0.05) for the right anterior cerebral artery and middle cerebral artery under basal conditions and for the right posterior cerebral artery during hypercapnia. Similarly, the cerebrovascular reactivity to carbon dioxide was always higher in patients than in controls, with a significant difference for the left anterior and the right middle cerebral arteries (P<0.05) and the right posterior cerebral artery (P<0.01). The broad overlap of cerebrovascular blood flow velocities and CO2 reactivities in patients and controls precluded identification of values diagnostic of migraine. Nevertheless, transcranial Doppler ultrasonography offers the opportunity to noninvasively monitor cerebral blood flow parameters and, therefore, represents a valuable tool for vascular research in migraine.