Slowing and attenuation of the dominant frequency of the electroencephalogram (EEG) are changes commonly used to detect cerebral ischemia. To assess the validity of this method, the EEGs recorded during 93 episodes of circulatory arrest in ten normothermic, lightly anesthetized patients undergoing implantation of automatic internal cardioverting defibrillators (AICDs) were visually inspected for change. The number of events recorded for each patient varied from 5 to 18 and was a function of the duration and success of AICD testing in each patient. In 82 of 93 (88%) episodes, EEG changes were identified, and occurred an average of 10.2 s after the last normal heart beat. Of these 82, 67 (82%) illustrated slowing and attenuation. However, 15 (18%) of the hemodynamic events showed changes not previously described as indicative of cerebral ischemia: 6 (7%) showed a loss of delta-wave activity and 9 (11%) showed an increase in the amplitude of theta activity. Time to onset of these unusual changes (10.6 and 9.2 s, respectively) was not significantly different from that for EEG slowing and attenuation (10.2 s). Five of the ten subjects showed more than one pattern of EEG change. There was no significant difference in the time to onset of EEG change among individual patients, and neither were there differences in patterns of change associated with particular anesthetic agents. These results indicate that in normothermic, lightly anesthetized individuals, cerebral ischemia may cause changes in EEG pattern other than slowing and attenuation of dominant frequencies. These alternative patterns should be recognized as indicative of cerebral ischemia when intraoperative EEG monitoring is performed.