The anatomy of the conduction system was studied in 8 patients who had isolated congenital complete heart block. Atrial-axis discontinuity was seen in 7 hearts from children born to mothers with anti-Ro antibodies. The anticipated area of the atrioventricular node was occupied by fibrous and adipose tissue. The heart from the only child whose maternal serum was anti-Ro negative had nodoventricular discontinuity of the conduction system. This difference in anatomy may have etiologic significance.