A heart with an atrioventricular septal defect is characterized by absence of the atrioventricular muscular and membranous septa, a common atrioventricular junction, an unwedged position of the aortic valve annulus and disproportionate inlet--outlet dimensions of the ventricular septum. The clinical and post-mortem findings are described of a case which had intact atrial and ventricular septa but had all the other anatomical hallmarks of atrioventricular septal defect. The problems in clinical diagnosis may be overcome by cross-sectional echocardiography.