We present the two first Spanish cases about the endodermic heterotopia of the atrioventricular node. This is an infrequent and probably congenital lesion, with a not well known histogenesis and associated with heart blocks and sudden death, which affects women in most cases. The first observation attaches a 55 years-old woman with an acute leukemia who died during the first chemotherapy treatment. The second observation affects a 25 years-old woman, with previous heart blocks, who died suddenly. A complete post-mortem study was done in both cases (including cardiac conduction system) with conventional histopathological study and extensive immunohistochemical panel. Ultrastructural study of first case was done. In both cases a multicystic and tubular lesion was found in the atrioventricular node region. It was lined by two or more cubic and polyhedral cell layers. In the cystic lumina an eosinophilic material, which corresponded to acid and neutral mucosubstances was found. The maximal lesion's diameter were: 0.9 and 1.3 cm, respectively. Immunohistochemically it was detected in both cases strong positivity for cytoqueratines, EMA, CEA and focally for chromogranin A and calcitonin. The immunohistochemical results in both cases express and endodermic differentiation (with neuroendocrine fenotipe).