Fifty-six endothelial-derived urinary bladder tumor samples collected from 26 animals with bovine enzootic hematuria were selected for immunohistochemical studies. Expression of factor VIII-related antigen (FVIIIra), CD31, muscle-specific actin, uroplakin III (UPIII), and the cell cycle-related proteins cyclin D1 and p53 was evaluated in hemangiomas, "hemangioendotheliomas" (a vascular tumor that histologically is intermediate in appearance between a hemangioma and a conventional hemangiosarcoma), and hemangiosarcomas. Although CD31 expression was seen in all endothelial tumors tested, FVIIIra was not expressed in poorly differentiated endothelial tumor cells from solid areas or in 7 muscle-invasive hemangiosarcomas. Cyclin D1 overexpression was seen in 53% of hemangiomas, 82% of hemangioendotheliomas, and 95% of hemangiosarcomas. P53 immunoreactivity was only seen in muscle-invasive hemangiosarcomas. The UPIII staining pattern, normally very intense on the apical aspect and cytoplasm of superficial urothelial cells, was altered in the urothelium in an estimated 25% of hemangiomas, most hemangioendotheliomas, and most hemangiosarcomas. In conclusion, CD31 is a better marker than FVIIIra in the characterization of bovine endothelial tumors. The cell cycle regulatory pathways involving cyclin D1 and p53 seem to be impaired in endothelial urinary bladder tumors, p53 immunoreactivity positively correlating with enhanced invasion.