We measured regional cerebral blood flow (rCBF) in adult ischemic-type patients with moyamoya disease and in patients with atherothrombotic middle cerebral artery occlusion (MCAO) to investigate cerebral hemodynamics in adult ischemic-type of moyamoya disease. In this study we measured rCBF and regional cerebrovascular response (rCVR) using acetazolamide by Xenon-enhanced CT. Our subjects consisted of 15 adult ischemic-type patients with moyamoya disease and 27 atherothrombotic stroke patients with proximal occlusion of the middle cerebral artery. The region of interest was conducted in the anterior cerebral artery, middle cerebral artery and posterior cerebral artery territories as well as basal ganglia regions. rCBF was preserved in all regions of patients with moyamoya disease. However, rCVR severely decreased in the anterior circulation territory in patients with moyamoya disease compared with those of MCAO. These results suggest that rCBF in the anterior circulation territory of adult ischemic-type patients with moyamoya disease is preserved by vasodilation of the cerebral arteries, while cerebral hemodynamic reserve capacity is severely reduced. The results indicated that basal moyamoya vessels are dilated. These findings may be one of the reasons why stroke occurs more frequently in adult than child patients with moyamoya disease.