In order to investigate the involvement of cyclooxygenase (COX)-2 in cell growth and invasion of oral cancer, a human epidermoid carcinoma cell line KB minimally expressing COX-2 protein was transfected with COX-2 cDNA and these activities were compared with mock-transfected KB in vitro and in vivo. KB/COX-2 clones showed a similar growth rate in vitro compared to KB/neo clones, but demonstrated significantly increased PGE2 production, cell migration and invasion. These KB/COX-2 clones markedly expressed MMP-9, pro-MMP-2 and activated-MMP-2 as compared to KB/neo clones in gelatin zymography. Western blot analysis showed that expression of MT1-MMP, Rho and Rac 1 in KB/COX-2 clones were stronger than that in KB/neo clones, but expression of TIMP-1 and TIMP-2 were weaker in KB/COX-2 clones than in KB/neo clones. When these cells were inoculated subcutaneously into nude mice, tumorigenicity and tumor growth were significantly elevated in KB/COX-2 tumors than in KB/neo tumors, and the gelatinase activity was much stronger in KB/COX-2 tumor tissues than in KB/neo tumor tissues in film in situ zymography. The orthotopic inoculation of cells to the oral floor showed that local invasion was pronounced in KB/COX-2 tumors. These results indicated that overexpression of COX-2 elevated tumorigenicity, tumor growth and invasion of human KB carcinoma cells via up-regulated MMP and Rho family small GTPases and down-regulated TIMP activities.