Twenty-nine patients with lesions of the neck, skull base, and cavernous sinus had test balloon occlusions of the internal carotid artery (ICA) to determine the feasibility of sacrifice of the artery. Only one patient (3.4%) showed evidence of cerebrovascular compromise. Sixteen patients who tolerated test occlusions went on to ICA sacrifice. Ten patients had permanent balloon occlusion (PBO) of the ICA for cavernous aneurysms or to "trap" carotid-cavernous fistulae (CCF). Complications occurred in three patients (30%) with permanent morbidity in one patient (10%). One patient with CCF had PBO of the proximal ICA only, resulting in an unstable neurologic state and ultimately in death. Two patients had resection of skull base tumors 2 and 6 days after PBO of the ICA. Both suffered strokes and one died. Three patients had surgical sacrifice of the ICA without PBO. Two of these patients suffered cerebral ischemia without permanent sequelae. We conclude that test occlusion of the ICA with clinical monitoring will miss a significant number of patients with inadequate cerebrovascular reserve. Sensitivity is improved by controlled reduction of systemic blood pressure during the test occlusion. Resection of a skull base tumor soon after PBO of the ICA should be done in a delayed fashion or preceded by extracranial-intracranial arterial bypass. Patients who have had the artery sacrificed should be monitored in an intensive care setting for 48 hours to avoid hypotension, which could cause cerebrovascular ischemia.