This study was undertaken to analyze the correlation between primary gross tumor volume (GTVp) and prognosis in patients with nasopharyngeal carcinoma (NPC) undergoing intensity-modulated radiation therapy (IMRT). Between February 2001 and December 2006, 305 patients with NPC treated with IMRT were analyzed retrospectively. GTVp was calculated from treatment planning CT scans. Univariate and multivariate analyses indicated that GTVp had a statistically significant correlation to local control, distant metastasis, and overall survival in patients with NPC, whereas T classification was not an independent prognostic factor. Among patients classified with N0-1 and N2-3, there were significant differences in the rates of distant metastasis between those with GTVp smaller and larger than 25 mL (p < .001 and p = .002, respectively). GTVp is highly significant in evaluating local control, distant metastasis, and overall survival of patients with NPC treated with IMRT. Therefore, it is recommended that GTVp be included in the new TNM classification system. Copyright © 2012 Wiley Periodicals, Inc.