The UICC classification (TNM) represents the validated standard tool to describe tumor extent and includes prognostic information on the probability of disease control. The American Joint Committee on Cancer (AJCC) stage grouping is based on the evaluation of treatment and outcome. Gross tumor volume (GTV) might be more relevant than pure description (TNM) or stage grouping as prognostic factor for local control in head and neck cancer (HNC). Based on the observation of GTV-correlated outcome in our initial HNC patient cohort treated with IMRT, we tested the hypothesis that the GTV is the most reliable predictive tool in HNC outcome. A GTV based volumetric staging system (VS) was introduced, using two volumetric cut-off values (15 and 70 cm3). VS, TNM, and AJCC stages were assessed and correlated with outcome following primary radiation in 172 HNC patients. Analyses were based on Kaplan-Meier survival curves. VS proved to be superior to the TNM/AJCC in predicting outcome. In addition, VS enabled to stratify high- and low-risk patients in advanced TN stages. GTV represented the most important prognostic indicator in HNC treated with IMRT and is recommended to be considered for therapeutic decisions and estimation of outcome.