Background: Radiation therapy is a therapeutic modality widely used for treatment of upper aerodigestive tract (UADT) neoplasms. However, its action is not restricted to tumor cells, and it may cause a variety of adverse reactions, including reduced jaw mobility. Material and Methods: A prospective cohort study was conducted to assess changes in jaw mobility in patients with UADT cancer undergoing radiation therapy. Results: Fifty-six patients completed the study. The results showed a significant reduction in mouth opening ( p <0.001), right lateral excursion ( p =0.038) and left lateral excursion ( p =0.035) of the jaw, a significant increase in the presence ( p <0.001) and severity of oral mucositis ( p <0.001), and a significant decrease in performance status ( p <0.001) after radiation therapy. Thirty-six patients (64.3%) exhibited reduction in mouth opening after treatment. The variables significantly associated with mouth opening reduction on bivariate analysis were: modification of diet ( p =0.037), radiation field ( p =0.024), presence of mucositis ( p =0.003), and reduction in performance status ( p =0.007). After adjustment by the multivariate model, the only variables that remained significantly associated with reduction in mouth opening were presence of mucositis ( p =0.018) and reduction in performance status ( p =0.47). Conclusions: These findings indicate that patients with upper aerodigestive tract cancer experience reduced jaw mobility after radiation therapy, which is strongly correlated with mucositis and reduced functional ability.