Abstract: Objective 　To investigate the significance of expression of cyclooxygenase-2 (COX-2) in human transitional cell carcinomas ( TCCs) of the urinary bladder and to evaluate the value of COX-2 expression in exfoliated urothelial cells in early diagnosis. Methods 　The expression of COX-2 in human TCCs of the urinary bladder was observed with immunohistochemical methods and COX-2 immunocytology were performed on exfoliated urothelial cells in the patient s with TCC and 30 subject s with normal urothelium. Results 　COX-2 specific proteins were found in 35 of 48 TCC specimens but not in cont rol urinary bladder tissues. There are statistical significances between the Ta-1>> and T224>> stage cases ( P < 0. 05), but no correlation was found between COX-2 and tumor grading. The expression of COX-2 was negative in all controls. The positive staining of COX-2 in exfoliated urothelial cells of patient s with TCC was 67. 5 %, which was significantly higher than the positive rate of cytopathology (37. 5 %) ( P < 0. 05), especially in low grade ( G1 ) and low stage ( Ta～T1) cancer. Conclusions 　The result s suggested that COX-2 overexpression plays a role in development and invasion of TCCs. COX-2 immunocytology had a higher specificity in detection of bladder tumors and therefore would be a good marker in early diagnosis.