A prospective study of 26 patients undergoing cardiac operation requiring cardiopulmonary bypass was undertaken to determine the incidence and etiologic factors leading to post-operative neuro-psychiatric complications. Detailed evaluation of the patients showed the neuro-psychiatric complications after surgery occurred in 8 of the 26 patients (30.77%). If primitive reflexes were included, the cases increased to 10 (38.46%). Delirium was the most common complication, which was noted in 6 cases (23.08%). Other manifestations were Babinshski sign (2 cases), coma and seizure (1 case), and major depressive symptoms (1 case). Factors evaluated were: age, sex, heart disease, disease severity, type of surgery, combined physical illness, duration of anesthesia, duration of surgery, duration of bypass, mean systolic BP during operation, and mean BP during bypass. The following factors tended to be related to the occurrence of neuro-psychiatric complications: 1) duration of bypass, 2) mean BP during bypass, and 3) mean systolic BP during operation.