Twenty-three instances of internal carotid artery occlusion occurring with minimal neurological deficit in 22 patients are described. Although each of these patients was referred to the neurosurgical service for evaluation for an extracranial-intracranial microvascular bypass procedure, complete arteriographic evaluations of their cerebrovasculature suggested that alternative methods should be the treatment of choice. For each patient reported the ipsilateral external carotid artery was demonstrated by angiography to be an important source of collateral blood supply to the cerebral hemispheres or retinae distal to the occluded internal carotid arteries. Ten patients with no significant atherosclerotic narrowing or ulceration of the external carotid artery have remained free of symptoms of cerebral ischemia for 6 to 40 months. In twelve patients who developed delayed recurrent cerebral or retinal ischemia ipsilateral to their internal carotid artery occlusion, there were found obstructive and/or ulcerative plaques involving the common and/or external carotid arteries. Thromboendarterectomy in 11 of these patients gave complete relief of ischemic symptoms during the 4 to 36 months of postoperative follow up. One of these 12 patients refused operation and went on to develop a major cerebral infarction. Angiographic identification of a functionally important external carotid artery ipsilateral to an internal carotid artery occlusion carries considerable prognostic and therapeutic significance.