A patient affected by familial hemiplegic migraine underwent Transcranial Doppler Sonography twice: the first during a spontaneous attack with right hemiparesis and aphasia, the second during a headache-free period. During the attack the following haemodynamic changes were seen: (a) bilateral increase in the middle cerebral artery and anterior cerebral artery blood flow velocities (this increase was more pronounced on the left side), (b) decreased systo-diastolic ratio and pulsatility index on the right side, (c) increased systo-diastolic ratio and pulsatility index on the left side. Our results indicate that during the attack in this familial hemiplegic migraine patient a diffuse vasoconstriction of the basal cerebral arteries developed. Moreover, Transcranial Doppler Sonography data suggest that a prolonged vasoconstriction of the peripheral arterioles could play a role in determining the neurological symptoms in this syndrome.