Our hypothesis was that the postoperative abnormalities in septal motion observed by M-mode echocardiography (MME) are due to changes of either ventricular shape or of total cardiac motion within the thorax. We obtained preoperative and early and late postoperative MME and two-dimensional echocardiograms on 25 patients undergoing cardiac surgery. No patient had coronary artery disease. All patients had normal preoperative septal motion; 11 patients retained normal (group N) septal motion on postoperative MME; 14 patients developed abnormal (group A) septal motion. Comparison of these two groups revealed that the group A patients had a greater degree of posterior epicardial motion toward the chest wall during systole. This indicates a greater anterior motion of the entire heart within the thorax, which produces the observed septal motion abnormality by carrying the septum forward passively as the whole heart moves anteriorly. This excessive forward cardiac motion may be due to fixation of the heart anteriorly by postoperative sternal-cardiac adhesions. No changes in left ventricular shape, size, or function were found to be associated with abnormal septal motion, nor was septal contraction impaired. Changes in intrathoracic cardiac motion are the probable cause of septal motion abnormalities after cardiac surgery.