A rare form of endocardial cushion defect is an atrioventricular (AV) canal-type ventricular septal defect with cleft mitral valve in the absence of a primum atrial septal defect. In two infants with Down's syndrome, preoperative delineation of this entity was accomplished. M-mode echocardiographic features included presence of two distinct AV valves, attachment of the anterior portion of the anterior leaflet of the mitral valve to the interventricular septum, and normal septal motion. Two-dimensional echocardiography demonstrated the intact atrial septum, ventricular septal defect, presence of two distinct AV valves, and the cleft mitral valve. Angled (axial) cineangiography was employed to delineate the AV valve morphological characteristics, position of ventricular septal defect, and absence of a primum atrial septal defect. Since the surgical approach to this lesion is different from other forms of endocardial cushion defects and specific problems related to AV valve morphology may be encountered, the preoperative diagnosis of this entity is important.