To evaluate the mechanism of paradoxical septal motion in patients with right ventricular volume overload (RVVO), short axis cross-sectional, echocardiographic studies of the left ventricle (LV) and interventricular septum (IVS) were performed in 19 patients with paradoxical septal motion due to RVVO and in 20 normal subjects. Short axis study in normal subjects revealed the left ventricle to be a relatively circular structure during both diastole and systole. In patients with RVVO a change in LC diastolic shape was observed. This change in shape varied from a slight flattening of the LV and IVS during diastole to total reversal of the normal direction of septal curvature such that the IVS became concave toward the RV and convex toward the LV. During systole the LV and IVS returned to their normal relatively circular configuration. This change in LV shape from diastole to systole resulted in net motion of the IVS toward the right ventricle (paradoxically). This study therefore suggests that paradoxical septal motion in patients with right ventricular volume overload is a result of a change in the diastolic shape of the left ventricle.