Hydranencephaly is a condition in which cerebral hemisheres are absent and reduced to fluid-filled sacs in a normal skull. Numerous causes have been proposed, but bilateral occlusion of the internal carotid arteries during early fetal development can explain most of the pathologic abnormalities. We evaluated a case of hydranencephaly by brain CT and magnetic resonance angiography. Magnetic resonance angiography showed flow within the vertebral and basilar arteries without internal carotid intracranial flow above the supraclinoid segment. A brief review of the related literature was given on this subject.