The purpose of this study was to determine whether preoperative angiographic patterns of collateral cerebral blood flow correlate with protection against intraoperative electroencephalographic (EEG) evidence of cerebral ischemia caused by carotid artery cross-clamping during carotid endarterectomy. Previous studies have shown that contralateral carotid artery occlusion and intracranial stenoses are associated with cerebral ischemia during carotid endarterectomy; however, the angiographic collateral flow patterns associated with cerebral ischemia have not been identified. This paper reports a retrospective study of 67 patients who underwent two- to four-vessel cerebral angiography followed by carotid endarterectomy with 16-channel EEG monitoring. The angiograms were reviewed for extracranial occlusive disease and collateral flow patterns, and the EEG recordings were analyzed for ischemic changes during carotid artery cross-clamping. Statistical analysis was by Fisher's exact test. Cross-filling of the anterior and middle cerebral arteries from the contralateral carotid artery through the anterior communicating artery correlated with a decreased incidence of EEG ischemic changes. Only 21% of patients with this collateral flow pattern showed ischemic changes compared to 50% of patients without this pattern (p < 0.03). Three angiographic findings occurring in combination on the side contralateral to surgery correlated with EEG ischemia: 1) occlusion of the contralateral internal carotid artery (five of seven or 71%, p < 0.03); 2) collateral flow from the external carotid circulation to the internal carotid circulation via the ophthalmic artery; and 3) collateral flow from the posterior circulation to the contralateral anterior circulation via the posterior communicating artery. The data presented here corroborate the correlation between contralateral carotid artery occlusion and cerebral ischemia during carotid endarterectomy. They also demonstrate that cross-filling of the anterior and middle cerebral arteries by the contralateral carotid artery protects against such ischemia. This collateral flow may serve as an indicator of tolerance to carotid artery cross-clamping.