To study the expression of cyclooxygenase 2 (COX-2) gene and its relationship with clinicopathological characteristics of lung cancer, expression of the COX-2 mRNA was evaluated by reverse transcription polymerase chain reaction (RT-PCR) in cancerous tissues and paired adjacent non-cancerous tissues from 56 patients and benign lesions from 12 patients. Our results showed that expression of COX-2 gene was detected in a significantly greater proportion of cancerous tissues (60.7%) than adjacent noncancerous tissues (10.7%, P<0.01) and benign lesions (3/12, P<0.05). Expression of COX-2 gene was higher in adenocarcinoma than in squamous carcinoma (P<0.01). There was no significant relationship between COX-2 gene expression and patients' age, sex, histological type of tumors, differentiation degree and TNM stages (P>0.05). The up-regulation of COX-2 gene in lung cancer tissues especially in adenocarcinoma suggested that COX-2 may play a role in the lung carcinogenesis and COX-2 gene may serve as a potential therapeutic target in lung cancer.