The possible determinants of a fatal outcome following surgical repair of the tetralogy of Fallot were analyzed by postmortem examination in 14 cases. Only one patient died late in the postoperative period, due to patch detachment; the others died shortly after undergoing surgery. Death could not be explained in three cases. In three patients with perimembranous ventricular septal defects, complete atrioventricular block occurred as a consequence of traumatic disruption of the branching bundle. In four cases residual stenosis of the pulmonary outflow tract was shown; in one of these cases adequate relief of obstruction had been prevented by a large conal coronary artery. A residual mitral cleft was found in one patient. Death was not related to the cardiac conditions in two cases; one of these patients died of brain apoplexy and the other of tracheal hemorrhage. These findings stress the need for accurate preoperative diagnosis and precise knowledge of the surgical anatomy of this malformation.