Alterations of intracranial vessel tone have been implicated in the pathophysiology of migraine. The cerebrovascular reactivity was measured by means of transcranial Doppler in 60 migraine patients with (n = 30) or without aura (n = 30) during the headache-free interval and in 30 healthy controls. The vasomotor response was evaluated during hypercapnia induced by inhalation of a mixture of CO2 5% and O-2 95% and during hypocapnia obtained after voluntary hyperventilation. To improve the power of the study in detecting possible abnormalities of cerebrovascular reactivity, two different measures were performed at 1 week intervals in migraine patients and controls. Reactivity index values during CO2 inhalation were significantly different (p = 0.01) among the three groups during the first and second measurements; in particular, lower values were found in patients suffering from migraine without aura with respect to controls (p < 0.05, Scheffe's test). Values of reactivity index obtained following induction of hypocapnia did not differ between migraine patients and controls (all p values > 0.05). Our data suggest a reduced vasodilatory response to hypercapnia of cerebral arterioles in patients suffering from migraine without aura with respect to controls that might be related to baseline arteriolar vasodilation.