On the basis of previous experience, we have analyzed the anatomy of a series of 53 hearts catalogued as having tetralogy of Fallot in the museum of the Children's Hospital of Pittsburgh. All hearts had a ventricular septal defect, aortic overriding, infundibular pulmonary obstruction, and right ventricular hypertrophy. None of the hearts had been operated upon during life. We paid particular attention to features of surgical importance. Thus the ventricular septal defect was found to be of variable form. It was perimembranous in 42 hearts. In 11 hearts it had entirely muscular rims and in the remaining heart it was roofed by the conjoined aortic and pulmonary valve rings (subarterial). From our previous histologic experience, it was evident that this varying morphology significantly affected the surgical anatomy of the atrioventricular conduction tissues. We endeavored to display this disposition as might be viewed by the surgeon. The degree of aortic override was variable, the aortic valve being connected by 15% to 95% to the right ventricle. In 17 hearts more than half the aortic valve was attached to right ventricular musculature. The nature of the infundibular obstruction also varied markedly. In all cases (except the one with absent infundibular septum) the infundibular septum was deviated in cephalad and anterior direction so as to produce obstruction. In the majority of hearts further anatomic structures also contributed to the obstruction. In some hearts there was hypertrophy of the infundibular septum, particularly at its junction with the trabecular septum. In others there was hypertrophy of the trabecula septomarginalis, and in many hearts there was hypertrophy of additional anterior infundibular trabeculations. The pulmonary valve was the narrowest point of the outflow tract in only six hearts, but in many of the others, it was abnormal. The results emphasize that while hearts may exhibit the classical features of tetralogy of Fallot, there is usually considerable individual variation in each of these features which is of major surgical significance.