Cyclooxygenase (COX)-inhibiting drugs have antitumor activity in canine and rodent models of urinary bladder cancer. Two isoenzymes of COX have been identified, COX-1 and COX-2. The purpose of this study was to characterize COX-1 and COX-2 expression in human invasive transitional cell carcinoma of the urinary bladder by immunohistochemistry and Western blot analysis. COX-2 was not expressed in normal urinary bladder samples but was detected in 25 of 29 (86%) invasive transitional cell carcinomas of the urinary bladder and in 6 of 8 (75%) cases of carcinoma in situ. These results indicate that COX-2 may play a role in bladder cancer in humans and support further study of COX- 2 inhibitors as potential antitumor agents in human bladder cancer.