Atrioventricular septal defects represent a class of congenital cardiac malformations that vary in presentation and management strategy depending upon the severity of the particular lesions present. We present the case of a premature neonate who had a partial atrioventricular septal defect and an accessory mitral (or left atrioventricular) valve leaflet. The latter caused severe left ventricular outflow tract obstruction and severely depressed left ventricular function. We found only one other report of this atrioventricular valve abnormality in association with atrioventricular septal defect. To our knowledge, our patient (at a body weight of 1,800 g) is the smallest to survive corrective surgery of an accessory mitral valve leaflet with severe left ventricular outflow tract obstruction. In addition to our patient's case, we discuss the relevant medical literature.