Cyclooxygenase- 2 (COX-2) is an inducible enzyme that is not found in normal conditions,. but is induced by a varie ty of pathophysiologic conditions of tissues by growth factors. inflammatory stimuli. oncogenes and tumor promoters, COX-2 is upregulated in a number of epithelial cancers. including in oral premalignant and malignant lesions, The mode of action of COX-2 in carcinogenesis may include mutiple mechanisms that may be acting at different stages of malignant disease, In this study. the expression of COX- 2 protein was assessed quantitatively 없d qualitatively by immunohistochemistry during DMBA-induced hamster buccal pouch carcinogenesis, The immunoreactivity for COX-2 protein increased as the tissue passed from hyperplasia to dysplasia and SCC, The highest mean expression was SCC at 14 week, The differences between COX-2 expression in the normal and that the dysplastic and carcinomatous lesions was statistically significant, In addition. the mean values of COX -2 expression in the stromal cells increased gradually during malignant progression, The results suggest that increased COX-2 expression may be associated with the chemically induced carcinogenic progression of hamster buccal pouch model, The gradual increasing COX-2 expression de tected during the progressive manner toward more malignant lesions shows that the COX-2 protein can have an important role in both the early and the later stages of multistep oral carcinogenesis