Congenital polycystic tumor of the atrioventricular node is a rare tumor of disputed histogenesis. It constant location in the atrioventricular node separates it from other cardiac cysts and tumors of indisputable endodermal origin. New studies of the embryogenesis of conduction tissues provide a reasonable explanation for the site of the lesion. Morphologic and histochemical features of the tumor fail to conclusively favor either a mesothelial or an endodermal genesis. A recently studied case provides some new morphologic observations (complex desmosomes, granule-containing epithelial cells) and suggests the possibility of of familial occurrence.