Abstract: The observational results on histology and immunohistochemistry of 5 adenomatoid tumors in epididymis showed that there were strong positive expressions for cytokeratin,epithelial membrane antigen(EMA)and vementin,and negativity for factor Vlll-related antigen(FV111RAg) in all tumor cells.These observations exhibited a obvious biphasic differentiated (epithelial and mesenchymal)nature of tumor cells,and indicated that adenomatoid tumors were of mesothelial origin.In differential diagnosis,adenomatoid tumor should be distinqushed from epithelioid(histocytoid)hemangioma and adenocarcinoma in epididymis.