Diagnosis of moyamoya diseases primarily depends on the angiographic demonstration of stenosis and/or occlusion of internal carotid bifurcation, and extensive collaterals. Nine patients with moyamoya disease were studied with high resolution computed tomography(HRCT), and the CT findings were retrospectively analyzed. The CT scans revealed not only parenchymal ischemic and hemorrhagic changes, but also mottled, tortuous, linear, and/or thalamic regions, corresponding to extensive collaterals on angiography, in all nine patients. In addition, the circle of Willis was poorly or partly visualized in eight patients, which seems to reflect the vascular occlusion or stenosis. HRCT appears to play an important role in the diagnosis of moyamoya disease.