We performed serial transcranial Doppler examinations on 35 patients undergoing 40 consecutive carotid endarterectomies (CEAs) to study alterations in collateral flow and middle cerebral artery (MCA) blood flow velocity. Ipsilateral peri-orbital collateral flow was abolished abruptly in all 16 cases. Compensatory anterior communicating artery collateral was abolished in 17 of 18 cases, and reversed in the other with a remaining severe contralateral carotid lesion. Post-operatively, ipsilateral MCA velocity increased significantly in 26 cases, decreased in 7, and remained unchanged in 7. Sixteen patients had transient hyperperfusion during the first week. Similar changes were observed on the contralateral side. There was no significant association between hyperperfusion and clinical or physiological variables except age. One patient had ipsilateral headache post-operatively associated with hyperperfusion. We conclude that CEA produces rapid alterations of collateral flow and may affect blood flow to both cerebral hemispheres. Transient hyperperfusion, although common, is seldom severe and rarely symptomatic.