Two autopsy cases of adult moyamoya disease were investigated by histometric and histopathologic techniques and the findings compared with those in three normotensive and five hypertensive controls in order to clarify the hemodynamic characteristics of the vertebrobasilar system in this disease. A thorough histologic examination of the vertebral and basilar arteries revealed that the fundamental arterial structures were well preserved in the vertebrobasilar system, unlike in the internal carotid system. A histometric study disclosed that the mean thickness of the media was considerably greater than that in comparable arteries of normotensive controls. From these observations, it was suggested that the essential arterial alterations in the vertebrobasilar system in moyamoya disease were hypertensive in nature and that this might be due to the increased pressure load on this system to compensate for the occlusive state of the internal carotid system. The medial hypertrophy in the vertebrobasilar system, even after massive medial atrophy in the carotid system, was impressive and suggestive of the complexity of hemodynamic changes and consequent tissue responses in this moyamoya disease.