Cyclooxygenase (COX) is a key enzyme in the synthesis of prostaglandins from arachidonic acid. Much evidence, including that from epidemiological and experimental studies, suggests that the inducible form of COX, COX-2, is increased in colon tumor tissues and is involved in colon cancer tumorigenesis. To determine the significance of COX-2 in tumorigenesis in the urinary bladder, the expression of COX-2 in transitional cell carcinoma and preneoplastic lesions of the bladder was examined. Tumor specificity of COX-2 immunoblotting was 100% in 12 of 35 (34%) tumors, but in 0 of the 10 normal urothelia samples. COX-2 expression was significantly correlated with tumor stage in 9 of 20 (45%) muscle-invasive (pT2-4) tumors and in 3 of 15 (20%) superficially invasive (pT1) tumors (P < 0.05). Immunohistochemical examination revealed that 13 of 14 (93%) samples of carcinoma in situ (CIS), which may be the precursor of muscle-invasive-type tumors, expressed COX-2, whereas 10 of 21 (48%) samples of dysplasia, which may be the precursor of both superficially invasive and muscle-invasive tumors, expressed COX-2. From the expression profile of COX-2 in these various urothelia, it is suggested that COX-2 is involved in the development of transitional cell carcinoma of the urinary bladder, especially that of muscle-invasive tumors via CIS. Furthermore, COX-2 may be a therapeutic target for CIS because of the high expression rate of COX-2 in CIS lesions.