Three hundred sixty-seven carotid endarterectomies were monitored using routine electroencephalographic (EEG) techniques. In 9.8%, changes in the EEG followed clamping of the internal carotid artery and could roughly be grouped into 6 patterns. The most common (47%) was rapid unilateral or bilateral attenuation of background anesthetic-induced fast EEG activity. Conclusions drawn from studies of the 9 patients who had immediate postoperative neurological deficits of varying degrees are presented as they illustrate monitoring techniques that are useful for early recognition of cerebral ischemia. Thirty-five patients were monitored with both routine and computerized techniques - the latter proved more useful than the former on all counts.