Previous studies have shown that migraine with aura is associated with the reduction of regional cerebral blood flow (rCBF). However, the question of whether the reduction of rCBF during migraine aura is caused by cerebral vasospasm or is secondary to the neural depression (spreading depression) is still disputed. We measured rCBF by high resolution SPECT method during the attack of migraine and examined whether the reduction in flow corresponds to the cerebral vascular territory. Fourteen patients with migraine with aura (7 men and 7 women, 34.7 +/- 17.8 years) were studied. In all the patients rCBF was measured during the interictal period and in four patients rCBF was measured during the aura of migraine. SPECT measurements of rCBF was performed using Tc-99m-PAO (740 MBq) as a tracer. During the aura of scintillation scotoma in the unilateral visual field rCBF was reduced in the opposite occipital, temporal and thalamic regions which corresponded clearly to the region of the posterior cerebral arterial territory. The reduction of rCBF was by 31 approximately 49% compared with the opposite hemisphere. Cerebral spinal fluid lactate level during the headache measured in one patient was higher (38 mg/dl) than the interictal period (12 mg/dl). Our data indicated that the reduction of rCBF during the aura is caused by ischemia probably due to the cerebral vasospasm and is not secondary to the neuronal depression. It was also suggested that the primary site of rCBF reduction during the visual aura is the occipital association cortex which is reported to be responsible for the visual hallucination.