Abnormal systolic interventricular septal motion is an echocardiographic manifestation of right ventricular volume overload. In order to determine the anatomical basis for this echocardiographic finding, septal motion recorded by left lateral or left anterior oblique ventriculograms was compared with echocardiography ventricular septal motion. Thirteen patients with secundum atrial septal defects and 7 control patients with trivial or no heart disease were included in the study. We found that on ventriculograms the systolic motion of the interventricular septum was similar in both atrial septal defect and control patients. That is, the cephalic third of the septum moved anteriorly in systole in 9 of the 13 patients with atrial septal defect and in 3 of the 7 controls. The caudal two-thirds of the septum moved posteriorly in all patients. These results are compatible with the theory that argues that the echocardiographic abnormalities of septal motion in patients with atrial septal defect result from an end-diastolic septal flattening or bowing of the septum into the cavity of the left ventricle.