5-Fluorouracil (5-FU) is a widely used drug in head and neck squamous cell carcinoma (HNSCC). In the anabolic pathway of 5-FU, the first step in activation of the drug is phosphorylation of 5-FU by orotate phosphoribosyltransferase (OPRT), which directly metabolizes 5-FU to 5-fluorouridine monophosphate (FUMP) in the presence of 5-phosphoribosyl-1-pyrophosphate. To date, OPRT expression in the tumors has been related to the clinical response or survival of cancer patients receiving 5-FU-based chemotherapy. In this study, we examined whether OPRT expression correlates with the chemosensitivity to 5-FU and cell proliferation in HNSCC. We constitutively expressed an OPRT cDNA in an HNSCC cell line. The effects of OPRT expression on in vitro cell growth and 5-FU cytotoxicity were examined. OPRT transfection increases the cytotoxicity of 5-FU without affecting cell proliferation of HNSCC cells in vitro. These results indicate that OPRT expression plays an important role in the sensitivity of HNSCC to 5-FU chemotherapy.