In 1929, one of us1reviewed the literature on congenital heart block and reported the first case of complete heart block of congenital origin studied by serial sections through the conduction system. One other case of congenital heart block in which there was partial (2:1) block had been studied in this manner by Wilson and Grant.2The case to be reported is the second case of congenital, complete heart block studied histopathologically. Congenital heart block is not common. In the review1in 1929, thirty cases were accepted from the literature as fulfilling certain diagnostic criteria. Many other cases were excluded because of insufficient proof either of their congenital origin or of the existence of heart block. The criteria of acceptance in certain instances may have been too rigorously applied. For instance, Calandre's3case was excluded because there was a possibility that the block was due to