A case of so-called mesothelioma of the atrioventricular node is presented. Controversy exists as to whether this lesion is of mesodermal or endodermal origin. The light and electron microscopic morphologic characteristics in this case were identical to those reported previously. The glandular component produced mucin that resisted digestion with both hyaluronidase and diastase; this staining pattern is characteristic of endodermal rather than of mesodermal tissue. Immunohistochemical methods demonstrated abundant carcinoembryonic antigen (CEA) in the cytoplasm of the cells composing the lesion. The presence of CEA strongly argues for an endodermal origin, since this antigen characterizes tissue derived from endoderm and is generally absent from mesoderm. The lesion probably represents endodermal foregut tissue that is displaced during embryogenesis. As such, it is not a true neoplasm. It is proposed that this lesion be designated "congenital endodermal heterotopia of the atrioventricular node."