Multiple observations in the laboratory and the clinical setting have linked expression of the enzyme cyclo-oxygenase-2 (COX-2) to carcinogenesis. The frequency and amount of COX-2 and Bcl-2 expression in primary lung and bladder cancer sites were detennined by immunoblot analysis of cell lysates prepared from frozen human tumor tissue and matched normal adjacent tissue. COX-2 protein was expressed statistically more frequently and at a higher level in primary adenocarcinomas and squamous cell carcinomas of the lung as well as transitional cell carcinomas of the bladder than in normal adjacent tissue. No correlation was observed between COX-2 and Bcl-2 expression in either the lung or bladder cancer specimens. Immunohistochemistry was also employed to localize COX-2 expression. In addition to expression in the malignant tissues, COX-2 was occasionally localized to the normal bronchial and transitional cell epithelia of the normal adjacent tissue. Detection of COX-2 in histologically normal appearing adjacent tissue suggests that COX-2 may be involved in early cellular changes leading to the development of lung and bladder cancer.