AbstractThe interatrial septum was studied with B‐mode echocardiography to determine whether there were specific ultrasonic features characteristic of such congenital heart diseases as atrial septal defect and endocardial cushion defect. In transverse tomograms in the third and fourth intercostal space the interatrial septum is detected almost parallel to the anterior chest wall. The septum continues leftward to the echo of the anterior mitral leaflet. When the interatrial septum extends beyond the left sternal border, it can also be recorded near the left sternal border in the sagittal tomogram. The interatrial septum can also be recorded conventional echocardiography. The pattern is wavy and synchronous with the heart beat with an amplitude of about 10 mm. It is seen near the chest wall in patients with left atrial enlargement and far from the chest wall in patients with right atrial enlargement. The defect in the interatrial septum was visualized B‐mode and conventional echocardiogram in patients with ostium secundum atrial septal defect. The defects were no longer observed after surg