Two cases of spontaneous internal carotid artery occlusion due to giant intracranial carotid artery aneurysm. Although spontaneous thrombosis of a giant intracranial aneurysm is relatively common, occlusion of its parent artery is rare. We describe two recent patients in whom the parent artery spontaneously occluded. One patient had severe stenosis of the left internal carotid artery, with delayed appearance of a faint shadow of vascular widening near the posterior clinoid process. One month later, complete occlusion of the left internal carotid artery was shown angiographically. The second patient had dysarthria and left hemiparesis, resulting in the diagnosis of a left internal carotid artery giant aneurysm. He had suffered an episode of visual disturbance of the right eye 5 years before. Angiography showed the right cervical internal carotid artery to be occluded. We believe the mechanism of parent artery occlusion in our two patients to be due first to stretching of the internal carotid artery by the enlarged aneurysm, followed by compression of the internal carotid artery by the aneurysm itself. Next, the anterior clinoid process and the optic nerve are involved, and, finally, thrombosis of the aneurysmal cavity extends into the internal carotid artery itself. 