Certain cardiovascular operations require cardiopulmonary bypass and prolonged circulatory arrest to provide an adequate operative field. Profound hypothermia is induced to protect brain function during these periods without cerebral perfusion. Because peripheral body temperatures may not be valid indicators of brain temperature, we conducted this study to establish an alternative, more reliable, method of determining the appropriate level of hypothermia. We measured peripheral body temperatures from 3 sites (rectum, nasopharynx, and esophagus) at the onset of hypothermic-induced electrocerebral silence (ECS) in the intraoperative electroencephalogram (EEG) of 56 adults undergoing cardiovascular procedures. The EEG in most patients showed a characteristic progression of changes from its normal resting state during cooling. Gradual depression and slowing of background rhythms was followed by generalized, periodic, slow-wave transients, prolongation of the intervals between the transients, and then the onset of ECS. In cases in which the slow-wave transients did not appear, the initial depression and slowing of background activity gradually progressed to ECS. No consistent relationship was found between peripheral body temperatures at various sites and the onset of ECS. Using EEG-guided hypothermia, there was low morbidity and mortality. These data suggest that ECS is a safe and reliable guide for determining the appropriate level of hypothermia during cardiovascular procedures.