The involvement of cyclooxygenase (COX)-2 in oral carcinogenesis and outcome of the patients is not fully understood. To determine whether COX-2 expression could serve as an indicator for them, we examined the expression of COX-2 and DNA topoisomerase (DNA-Topo) II alpha as an index of cell proliferating activity in precancerous and cancerous lesions of the oral mucosa. A 164 samples composed of 60 intraepithelial dysplasias (IEDs), 12 carcinomas in situ (CISs), 72 squamous cell carcinomas (SCCs) including 12 early invasive SCCs, 10 undifferentiated carcinomas (UCs), and 10 epithelial hyperplasias (EHPs) in the oral mucosa were examined immunohistochemically for COX-2 and DNA-Topo II alpha. Normal squamous epithelium as the control showed no COX-2 expression, whereas 41% of IEDs, 67% of CISs, 74% of SCCs, and 86% of UCs demonstrated increased COX-2 expression with elevated DNA-Topo II alpha labeling index (LI). High COX-2 expression was also observed in 61% of EHPs, but DNA-Topo II alpha LI was very low. Increased expression of COX-2 protein correlated with elevated DNA-Topo II alpha LI, indicating that COX-2 may contribute to malignant transformation and tumor growth. These two enzyme activities were increased as T, N, and M categories and stages proceeded. The patients with high expression of both COX-2 and DNA-Topo II alpha showed poor prognosis. Our results suggested that COX-2 expression become a possible indicator in oral carcinogenesis and may reflect the outcome of the patients.