RESUMEN
Adenomatoid tumors are well-recognized neoplasms generally to be of mesothelial derivation. We experienced a case of an adenomatoid tumor of the tail of the epididymis in a 56-year-old male. Grossly the tumor was firm and whitish gray, and microscopically it consisted of glandular, cord-like, microcystic structures which were lined by flattened endothelial like to plump cuboidal cells. Immunohistochemical stains whowed positivity for keratin and negativity for facter VIII related antigen and carcinoembryonic antigen. Ultrastructually, there was many long microvilli projecting into the glandular lumina and intracytoplasmic luminal spaces, desmosomes, and prominent cytoplasmic tonofilaments. Those findings strongly support the mesothelial origin of the adenomatoid tumor especially in the glandular type. It also lead us to suggest that the term adenomatoid tumor should be remain in use for light microscopic diagnosis, and that the term adenomatoid mesothelioma should be applied when the mesothelial nature of an adenomatoid tumor is proven by electron microscopy and immunohistochemical stains.