A prospective study of the conditions of 100 open-heart surgery patients elicited more precise information about incidence, characteristics, and causes of central nervous system dysfunctions after surgery. Neurological, psychometric, and behavioral observations were made preoperatively and postoperatively. Half the sample developed neurological damage following recovery from anesthesia. Forty-three percent of survivors developed behavioral abnormalities preceded by focal neurological damage; intellectual functions were depressed in all such patients. In most patients neurological signs and mental symptoms disappeared and mental status normalized by discharge. However, 15% of survivors had signs of cerebral damage at discharge. Cerebral damage was significantly related to increasing age and depression of arterial pressure. In patients with cerebral dysfunction, arterial pressure fell to levels which did not support adequate cerebral perfusion. Older patients were especially vulnerable to such effects. Neuropathological findings suggested cerebral ischemia.