Profound hypothermia protects cerebral function during circulatory arrest in the surgical treatment of a variety of cardiac and aortic abnormalities. Despite its importance, techniques to determine the appropriate level of hypothermia vary; studies of temperatures recorded from multiple peripheral body sites show inconsistent findings. The purpose of this study is to establish objective criteria to consistently identify intraoperatively the safe level of hypothermia. Our studies are based on experimental evidence showing a correlation between brain temperature and development of electrocerebral silence (ECS) on the electroencephalogram (EEG), and the recognition that the EEG, as an objective measure of brain function, can easily be recorded intraoperatively. We studied 56 patients who required circulatory arrest during operation for replacement of the ascending aorta or aortic arch (N = 55) or aortic valve replacement (N = 1). Peripheral body temperatures from the nasopharynx, esophagus, and rectum and the EEG were continuously recorded during body cooling. Circulatory arrest time ranged from 14 to 109 minutes. No peripheral body temperature from a single site or from a combination of sites consistently predicted ECS. There was a wide variation in temperature among body sites when ECS occurred: nasopharyngeal, 10.1 degrees to 24.1 degrees C; esophageal, 7.2 degrees to 23.1 degrees C; rectal, 12.8 degrees to 28.6 degrees C. Fifty-one (91%) of the 56 patients survived. Three had neurological deficits, none clearly related to hypothermia. Two patients (3.6%) required reexploration for postoperative bleeding. We conclude that monitoring the EEG to identify ECS is a safe, consistent, and objective method of determining the appropriate level of hypothermia.