The reactivity of posterior cerebral artery (PCA) to light stimulation was studied with transcranial Doppler in 9 migraine patients (5 with aura, 4 without aura) within six hours of the onset of headache (mean 115 ± 78 minutes). The PCA mean flow velocity was recorded while the patient was in the dark and with the eyes closed and, subsequently, while the light was on and with the eyes open. Both symptomatic and asymptomatic sides were recorded. The same test was repeated thirty minutes after injection of sumatriptan 6 mg sub-cutaneously and again on the third day after cessation of the attack. Photoreactivity was calculated, for each side and for each situation, as percent relative increase of the corresponding averaged mean flow velocity recorded in darkness. The results showed that, although absolute flow velocities were not statistically different between sides and situations, photoreactivity was significantly increased on the symptomatic side during attacks (+28% vs 15% respectively). After sumatriptan, photoreactivity became symmetrical and (nonsignificantly) inferior to the values recorded in the interictal period. These findings suggest that, during headache, peripheral resistance branches of PCA on the symptomatic side are hypersensitive to vasodilatory stimuli. This could represent the counterpart of the clinical scotomata and photophobia commonly experienced by migraine patients during attacks.