Osteoradionecrosis (ORN) is one of the most serious complications of head and neck radiotherapy and is considered a public health problem worldwide. This study aims to determine the prevalence and associated factors of ORN in patients undergoing radiotherapy for head and neck malignancy. A cross-sectional retrospective study was conducted, in which all medical records of patients undergoing head and neck radiation in the period between 2006 to 2015 (10 years) were examined. Clinical and demographic data were extracted. Multivariate Poisson regression analysis with robust variance was employed to access the relationship between ORN and independent variables (p<0.05; 95% CI). The sample comprised 413 medical records of patients undergoing radiotherapy. The prevalence of ORN was 9.7 %. Most participants were males (78.2%). The mean age of subjects was 55 years (± 14 years). The mandible was the main site of occurrence of ORN (85.0%). The following variables were associated with ORN : presence of oral mucositis (PR = 3.03; 95% CI: 1.30-7.03), history of smoking (PR = 0.23; 95% CI: 0.07-0.74), number of teeth removed before radiotherapy (PR = 1.06; 95% CI: 1.01-1.11) and visit to the dentist before radiation (PR = 0.08; 95% CI: 1.02-1.11). The prevalence of ORN was low and was associated with the presence of oral mucositis and the number of removed teeth before radiation. Visiting the dentist before radiotherapy and stop-ping smoking were protective factors for ORN.