Backgrounds: Abnormal motion of interventricular septum(IVS) is frequently observed even after uncomplicated cardiac surgery, but etiology is uncertain. Several studies using gated myocardial SPECT or echocardiography have been variously proposed that abnormal motion of IVS is related to ischemic injury to the septum during coronary artery bypass surgery , to the effect of cardiopulmonary bypass, or to increased anterior motion of the entire heart, to open pericardium. The purpose of this study is to determine which mechanism is associated with postoperative abnormal septal motion using various MR imaging techniques. Material and Methods: 18 patients (mean age: 58짹12 years, 15 males) were studied with MR imaging before and after coronary artery bypass surgery. All patients had normal rest/stress perfusion in septum without evidence of non-viable myocardium before surgery. Follow-up MRI including T2-weighted image, rest/stress perfusion and cine images, delay enhancement and myocardial tagging was done at 3 months after surgery. Results: Postoperatively, reduced septal motion at rest/stress state was seen in 10 patients. (56%) But, all patients showed normal T2-weighted image, normal rest/stress perfusion, and viable myocardium in septum. In group of apparently reduced motion of IVS, LV wall thickening was not different among regions. (P=0.13) Circumferential shortening of septum(-18.89 짹 5.41 vs -13.41짹 3.7; p<0.05) was even larger than of lateral wall. But, radial thickening of septum to LV centroid was smaller and even paradoxical compared with lateral wall. (-4.12짹 1.25 vs 17.74 짹 5.0 ; p<0.01) Conclusions: Abnormal motion of interventricular septum(IVS) is frequent finding after coronary artery bypass surgery. These results supported the hypothesis that it was caused not by the result of septal ischemic injury, but by recuitment of the IVS toward the RV to compensate impaired RV free wall motion after coronary artery bypass surgery.