One of the cyclooxygenase (COX) isoforms, COX-2, is overexpressed in various human cancers. In this study, we examined the gene expression levels of COX-2 in primary non-small cell lung cancers (NSCLC), metastatic lymph nodes, and normal lung tissues. The expression levels of the COX-2 gene were assessed by means of the reverse transcription polymerase chain reaction in 76 autopsy samples (29 primary NSCLC, 29 corresponding normal lung tissues, and 9 metastatic lymph nodes). The expression levels in NSCLC (both adenocarcinomas and squamous cell carcinomas) were significantly higher than in normal lung tissues and were significantly higher in adenocarcinomas than in squamous cell carcinomas. Differences between the levels of expression of COX-2 in primary tumors and their corresponding metastatic lymph nodes in 9 adenocarcinoma patients were not significant. Our results indicate that COX-2 may be associated with carcinogenesis of NSCLC, and that it may be a target for the treatment of NSCLC.