Oral cancers frequently contain DNA of human papillomavirus (HPV) type 16 or 18, but the functional significance of this is unclear. A role for HPV in the progression of oral cancer would be more plausible if the viral transforming genes were likely to be overexpressed in oral cancer cells. We therefore isolated and sequenced the long control region (LCR) of HPV-16 or HPV-18 from three oral cancer cell lines and two lines of HPV-16-immortalized oral keratinocytes, using PCR. The functional activity of each LCR was measured by cloning it into a luciferase expression vector, followed by transfection into both normal oral keratinocytes and oral cancer cells. For comparison, the LCRs of the wild-type HPV-16 and HPV-18 were studied. Several mutations were found in the LCRs isolated from oral cancer cells and HPV-immortalized oral epithelial cells. The promoter activity of the mutated LCRs was significantly higher than that of the equivalent wild-type LCRs in oral cancer cells that contained the same HPV type. These results imply that mutations in the LCR of HPVs in oral cancer could lead to increased expression of HPV-transforming proteins, which might contribute to the carcinogenic process.