OBJECTIVE To determine whether stump pressure changes significantly over time during temporary internal carotid artery (ICA) occlusion via an endovascular balloon, and to examine correlations between stump pressure changes and cerebral blood flow (CBF) as measured by xenon CT. METHODS Seventy candidates for ICA sacrifice were evaluated preoperatively with a test occlusion of the cervical ICA using an endovascular balloon. Measurements of ICA stump pressure above the occlusive balloon through a distal lumen in the balloon catheter were made throughout the 15-minute test. During occlusion, CBF was measured with stable xenon CT techniques. Patients were then categorized according to CBF results. RESULTS The stump pressure changed significantly (P < .001) during the occlusive period. On average, stump pressure increased 7.7 mmHg (12%). Patients determined by xenon CT to have an asymmetric decrease in CBF with balloon occlusion did not display the upward trend in stump pressures and were significantly different (P = .013) in that respect from patients who demonstrated either no CBF change or only minimal bilaterally symmetric decreases in CBF. CONCLUSIONS Large overlaps among the groups prohibit the use of stump pressure changes as an accurate predictor of CBF or of stroke risk in an individual patient. However, these data suggest an autoregulatory mechanism in the cerebral circulation that evolves over a period of minutes in contrast to the traditional concept of cerebral autoregulation which is thought to occur in a matter of seconds.