Transcranial Doppler (TCD) examinations were performed in 13 patients with common and 5 patients with classic migraine during attacks and compared to TCD findings during the headache-free period. Two distinct patterns of flow changes were detected to distinguish common from classic migraine on the basis of TCD findings. During attacks, patients with common migraine exhibited reduction of flow velocities associated with an increase of pulse wave amplitudes. Vascular bruits that were heard during the headache-free interval often disappeared. Opposite changes were found in attacks of classic migraine during the headache phase with increase of flow velocities, decrease of pulsatility and more prominent or newly appearing bruits. These findings were often diffuse and did not appear to correlate with side of headache or side of neurological aura. Uniform changes occurred in the cervical internal carotid artery and the basal cerebral arteries in either form of migraine. We propose that these changes represent caliber fluctuations of the large arteries, suggesting vasodilatation during attacks of common migraine and vasoconstriction during attacks of classic migraine. We do not intend to imply a casual role of these preliminary findings in migraine pathogenesis, but we suggest that TCD be used in combination with other methods to study vascular changes in migrainous disorders.