BACKGROUND: Abnormal interventricular septal motion (ASM) is frequently observed after open heart surgery (OHS). The aim of this study was to investigate the incidence and temporal change of ASM, and its underlying mechanism in patients who underwent OHS using transthoracic echocardiography (TTE). METHODS: In total, 165 patients [60 +/- 13 years, 92 (56%) men] who underwent coronary bypass surgery or heart valve surgery were consecutively enrolled in a prospective manner. TTE was performed preoperatively, at 3-6-month postoperatively, and at the 1-year follow-up visit. Routine TTE images and strain analysis were performed using velocity vector imaging. RESULTS: ASM was documented in 121 of 165 patients (73%) immediately after surgery: 26 patients (17%) presented concomitant expiratory diastolic flow reversal of the hepatic vein, 11 (7%) had inferior vena cava plethora, and 11 (7%) had both. Only 2 patients (1%) showed clinically discernible constriction. ASM persisted 3--6 months after surgery in 38 patients (25%), but only in 23 (15%) after 1 year. There was no difference in preoperative and postoperative peak systolic strain of all segments of the left ventricle (LV) between groups with or without ASM. However, systolic radial velocity (V(Rad)) of the mid anterior-septum and anterior wall of the LV significantly decreased in patients with ASM. CONCLUSION: Although ASM was common (74%) immediately after OHS, it disappeared over time without causing clinically detectable constriction. Furthermore, we consider that ASM might not be caused by myocardial ischemia, but by the decreased systolic V(Rad) of the interventricular septum after pericardium incision.