A case of a giant aneurysm arising from the basilar artery is presented. The patient a 41--year-old female, had recent memory disturbance, miscalculations and hemiparesis on admission. Vertebral angiograms displayed a giant aneurysm measuring approximately 66 mm in diameter arising from the basilar artery, a tortuous vertebral artery, an extremely dilated basilar artery and an agenesis of the left carotid artery. Due to the enormous size of the aneurysm and its location in the deep intracranial portion, radical surgery was abandoned. Six months following her discharge, she died. At autopsy, the giant aneurysm arising from the top of the basilar artery was found, existing just beneath the third ventricle and extending to the left basal ganglia. The lateral ventricle, the third ventricle and the Sylvian fissure were filled by hematoma resulting from the rupture of the aneurysm. It is possible that this giant aneurysm could be considered a vascular anomaly, and hence be distinguished from saccular aneurysms.