Cyclooxygenase-2 (COX-2) is reported to play an important role in carcinogenesis. We examined COX-2 expression in patients with breast cancer and benign breast tumors. Immunohistochemical staining revealed a high level of COX-2 expression in malignant lesions of invasive ductal carcinoma (IDC) at a rate of 40% and a low level of expression in 15% of adjacent normal-appearing breast epithelia. Similarly, in ductal carcinoma in situ (DCIS), a high level of COX-2 expression was found in 80% of malignant lesions and a low level of expression in 50% of normal epithelia. Reverse transcriptional polymerase chain reaction (RT-PCR) performed in 7 of these cases disclosed that COX-2 expression was restricted to the malignant lesion. Further, all 10 cases of fibroadenoma and 10 cases of intraductal papilloma, both of which are benign tumors, had a high level of COX-2 expression. When overexpression of COX-2 was analyzed in relation to the clinicopathological features of the patients, no characteristic correlation was noted. Our results demonstrated that COX-2 is expressed in mammary tissue during tumorigenesis of the breast gland, suggesting that the cyclooxygenase isoenzyme may be a target for the prevention and treatment of breast cancer.