Bovine urinary bladder tumors occur frequently in animals suffering from chronic enzootic hematuria because of prolonged ingestion of bracken fern (Pteridium spp.). Cyclooxygenase (COX) genes (COX-1 and COX-2) are known to be involved in the carcinogenesis of human and some animal urothelial tumors. The aim of the present study was to investigate COX-1 and COX-2 expression by immunohistochemical methods in 20 bovine urothelial carcinomas collected at public slaughterhouses from cows that had been suffering from chronic enzootic hematuria. COX-1 immunostaining was identified intracytoplasmically in normal urothelium and in 15 of 20 neoplastic specimens. COX-1 immunosignal in the tumor cells was either absent or weak. COX-2 was also expressed intracytoplasmically in 17 of 20 urothelial carcinomas. Moderate to intense COX-2 labeling was detected in both noninvasive and invasive urothelial carcinomas. Coexpression of both enzyme isoforms was also revealed by confocal laser scanning microscopic investigations. This study indicates that COX-2 is overexpressed in naturally occurring urothelial carcinomas of cows.