Congenital conduction defects of the heart have heretofore been grouped heterogenously in the literature under the broad term "congenital heart block." This title is somewhat inappropriate, since it connotes a basic physiologic rather than an anatomic disorder. Because the atrioventricular block is actually the result of an underlying anatomic defect, it would probably be preferable to classify this condition anatomically as with other congenital heart lesions, rather than physiologically. When malformations involve the cardiac conduction system, gross anatomic heart defects, according to published data, are extremely common.1-15However, in rare instances the pathology has been reported to be only microscopic.15-20Since modern diagnostic methods now enable us to establish ante mortem the precise nature of anomalies of this type, we can separate with a reasonable degree of accuracy those patients with purely microscopic lesions from those in whom there are complicating gross anatomic defects of the heart. The