The aim of this cohort study was to examine the role of chemokine (C-X-C motif) ligand 9 (CXCL9) on oral cavity squamous cell carcinoma (OSCC). Sera from 181 OSCC patients, 231 healthy individuals, and 50 OSCC tumor samples were enrolled. CXCL9 expression in tissue samples was analyzed by quantitative real-time PCR and immunohistochemistry. CXCL9 serum concentrations were measured by enzyme-linked immunosorbent assay. Effects of CXCL9 on OSCC cell function were investigated by cell proliferation assays, trans-well migration/invasion assays, and RNA interference. CXCL9 expression was significantly higher than for normal epithelium in the tissue samples. CXCL9 serum concentrations were also significantly higher in OSCC patients compared to those in healthy individuals. Serum CXCL9 levels were significantly higher in OSCC patients with higher pT status, pathological overall stages, tumor depths, and positive bone invasion (P = 0.033, 0.004, 0.041, and 0.002, respectively). Moreover, OSCC patients with higher CXCL9 levels (> 209 pg/mL, median level) before treatment had worse prognoses for overall survival and disease-specific survival (P = 0.0006 and 0.0009, respectively). Multivariate logistic regression analyses also indicated that higher CXCL9 serum levels were an independent prognostic factor for overall survival and disease-free survival (P = 0.003 and 0.004, respectively). The in vitro suppression of CXCL9 expression in SCC25 cells using specific interfering RNAs attenuated cell proliferation, migration and invasiveness. Our study demonstrated that CXCL9 is associated with tumor burden and aggressiveness of OSCC tumors and serum level of this ligand may be useful as a prognostic indicator. Copyright © 2013. Published by Elsevier Ltd.