Atrioventricular node congenital polycystic tumor, a lesion that causes heart block and sudden unexpected death, has been postulated to arise from the lymphatics, foregut endoderm, cardiac mesoderm, and mesothelium since its initial description in 1911. No theory concerning the histogenesis of this tumor has been completely satisfactory, either from an embryologic or a clinical standpoint. Biologic studies including cell culture systems from the lesions as well as optimal ultrastructural examination have been compromised due to postmortem autolysis. We studied four examples of this entity by standard histochemical stains for mucopolysaccharides, and by modified immunohistochemical labeling for carcinoembryonic antigen, factor VIII, and keratin. Results suggest an origin from foregut endoderm. The lesion has not been reported to be outside the region of the atrioventricular junction, but was demonstrated in the thymus gland in one of our cases.