BACKGROUND.: The purpose of this study was to assess predictors of mucositis in oropharyngeal and oral cavity cancer after definitive or adjuvant volumetric modulated arc radiotherapy (VMAT) +/− chemotherapy. METHODS.: Fifty patients were evaluated. Statistical analysis was performed for the following parameters as potential predictors of mucositis ≥ G2: total oral mucosa and oral mucosa minus target high-low radiation dose regions (planning target volumes [PTVs]), mean dose (Dmean), maximum dose (Dmax), chemotherapy, weight loss, and dysphagia. RESULTS.: Mucositis ≥ G2 was found to be statistically related to chemotherapy, weight loss, dysphagia ≥ G2, total oral mucosa Dmean ≥50 Gy and Dmax ≥65 Gy, V45 Gy >40%, V50 Gy >30%, and V55 Gy >20% of the oral mucosa minus target PTVs. A ratio between total oral mucosa and oral mucosa minus target PTVs >2.5 is related to G3 mucositis (p = .03). CONCLUSION.: New parameters were found as predictors of moderate-severe mucositis. © 2015 Wiley Periodicals, Inc. Head Neck 38: E815–E819, 2016