Malignant mesothelioma of the tunica vaginalis testis is an extremely rare tumor with 41 previously reported cases. The histological and immunohistological features of a new case in an 80-year-old patient are described and compared with an adenomatoid tumor of the tunica vaginalis testis, which is considered to be the benign variant of malignant mesothelioma. Both tumors revealed strong cytoplasmic staining for a panepithelial antibody (Lu-5) and membranous staining for BMA-120 (a mesothelial/endothelial cell marker) but yielded negative staining results with the endothelial cell markers QBend-10 (CD 34), Factor VIII-related antigen (vWF) and UEA-1. There was also negative staining for CEA, Ber-EP4, HEA-125 and Blood group related antigens A, B, H. An identical staining pattern was evident in normal and reactive mesothelial cells. Our data support a mesothelial rather than an endothelial derivation of the adenomatoid tumor studied.