Abnormal interventricular septal motion develops in the majority of patients after cardiac surgery and has been attributed to a variety of causes. This study assesses the role of cardiopulmonary (CP) bypass in the genesis of abnormal septal motion. Patients (22) having single coronary artery bypass grafts had septal motion and ejection fraction evaluated preoperatively and postoperatively. The results for 11 patients who had coronary artery bypass grafting (CABG) with CP were compared to those for 11 patients who had grafting without CP bypass. Postoperatively, all 11 patients having bypass grafting with CP bypass had diminution in septal function while 10 of 11 patients having bypass grafting performed without CP bypass had no change or improvement in septal motion (P < 0.0005). Changes in ejection fraction were not statistically significantly different. Evidently, abnormal septal motion in patients having CABG is related to CP bypass and/or myocardial preservation techniques.