M-mode echocardiograms obtained using the subxiphoid position of the transducer were compared with those obtained using the parasternal position in 21 children with atrioventricular defects. In the 16 children with complete atrioventricular defect, it was always possible to demonstrate a common valve orifice wholly occupying the space between the anterior and posterior cardiac walls with no interventricular septal tissue in the plane of the orifice. Scanning between the plane of this orifice and the ventricular cavity demonstrated a ventricular septum in all cases, separating components of the atrioventricular valve apparatus from each other, corroborating the impression that the valve was straddling the ventricular septum and, therefore, that the valve was common to both ventricles. No septal tissue was detected in scans ranging between the aortic valve and the common orifice in these 16 children. In the five children with partial atrioventricular defects, two atrioventricular orifices separated by septum could be demonstrated. The subxiphoid approach in these 21 children defined the anatomy of atrioventricular defects more clearly and more readily than the parasternal approach.