Background: We evaluated the prognostic role of gross tumor volumes (GTVs) of primary tumor and positive lymph nodes on overall survival (OS) and progression-free survival (PFS) in locally advanced unresectable sinonasal cancer (SNC) treated with definitive intensity-modulated radiotherapy (IMRT) with or without chemotherapy. Methods: Primary tumor GTV (GTV-T), pathologic neck nodes GTV (GTV-N), and positive retropharyngeal nodes GTV (GTV-RPN) of 34 patients with epithelial nonglandular SNC receiving IMRT with or without chemotherapy were retrospectively measured. The GTV variables were analyzed in relation with OS and PFS. Survival curves were estimated using the Kaplan-Meier method and compared with the log-rank test. We also estimated the crude cumulative incidence of locoregional relapses only. The optimal volume cutoff value was determined using an outcome-oriented method among the observed values. Results: GTV-T was significantly associated with decreased OS ( P=0.003) and PFS ( P=0.003). Moreover, patients with disease total volumes (GTV) smaller than 149.44 cm³ had better OS and PFS than patients with higher volumes ( P<0.0001 for both). Neck nodal metastasis impacted on OS and PFS ( P=0.030 and P=0.033, respectively), but GTV-N did not ( P=0.961; P=0.958). Retropharyngeal nodes metastasis was not associated with prognosis (OS: P=0.400; PFS: P=0.104). When GTV-RPN was added to GTV-N (GTV-TN), a relation with PFS ( P=0.041) and a trend toward significance for OS ( P=0.075) were found. Conclusions: Our results show that tumor volume is a powerful predictor of outcome in SNC. This could be useful to identify patients with worse prognosis deserving different treatment strategies.