The cross sectional echocardiograms of 150 children with atrioventricular septal defects were reviewed to determine the spatial orientation of the ventricles. In 125 cases with usual atrial arrangement (situs solitus), ventricular topology showed the right hand pattern. Of the 25 patients with atrial isomerism, 13 had similar right hand topology but 12 (48%) had the left hand pattern. This finding was more common in hearts in the right side of the chest (dextrocardia), but there was no significant correlation between ventricular topology and number of atrioventricular valves or the ventriculoarterial connection. In part because of inferences for disposition of the conducting tissue, knowledge of ventricular topology is important when operation for atrioventricular septal defects is considered. Ventricular topology can be promptly recognised by cross sectional echocardiography.