Internal carotid artery (ICA) back pressure correlates collateral cerebral blood flow, providing a critical pressure necessary for carotid endarterectomy without a shunt. The ICA back pressures were measured in 43 patients undergoing 50 endarterectomies under regional anesthesia. A four-minute ICA test occlusion was used to determine need for shunting. Mean ICA back pressure was 69 ± 2 (SE) mm Hg in 22 procedures (group 1), 34 ± 2 mm Hg in 25 procedures (group 2), and 15 ± 3 mm Hg in three procedures (group 3). Differences between groups were significant (P <.05) without significant (P>.05) differences in systemic pressure. Group 3 patients did not tolerate temporary carotid occlusion, confirming 25 mm Hg as the lower limit of adequate collateral flow. Five patients developed neurological complications, three in group 1 and two in group 2. Embolization may have occurred in two patients. However, some patients require more than minimum collateral flow and consequently need ICA back pressures higher than 25 to 50 mm Hg.