The development of paradoxical interventricular septal motion is a common consequence of cardiopulmonary bypass operation. The reason for this postoperative abnormal septal motion is not clear. 41 patients were studied preoperatively and postoperatively with radionuclide blood pool scan to evaluate the frequency of development of paradoxical septal motion with right ventricular volume overload before surgery and the frequency of development of paradoxical septal motion after cardiac surgery with cardiopulmoaary bypass, and to evaluate the change of EF related to the development of paradoxical septal motion after cardiac surgery. The results were as follows; 1) 7 of 41 patients with right ventricular volume overload (that is 17%) showed paradoxical septal motion before surgery. But 13 of 34 patients (that is 42%) had paradoxical septal motion after cardiac surgery with cardiopulmonary bypass. So open heart surgery with cardiopulmonary bypass related the development of paradoxical septal motion after surgery. 2) EF significantly decreased in patients who developed paradoxical septal motion after surgery, whereas the EF did not change in the patients who retained normal interventricular septal motion after surgery. So paradoxical septal motion usually reflected some diminution of left ventricular function, immediately after cardiac surgery.