PURPOSE This study focuses on the clinical characteristics of patients with osteoradionecrosis (ORN) of the jaws and on the relation between the extent of the ORN and the field of irradiation. PATIENTS AND METHODS The study group consisted of 80 patients referred for treatment of ORN. Charts of the extent of ORN, based on panoramic radiographs, were drawn by the clinical investigator and compared with charts of the field of irradiation drawn by the oncologists. Also recorded was diagnosis of the lesion, stage, location, treatment schedule, and period and dose of irradiation. Also documented were various potential initiating factors for ORN. RESULTS Smoking habits of ORN patients were similar to those of other patients with head and neck cancer. A new primary tumor or a recurrence was diagnosed in 10% of the patients at the time of ORN. Only 3 patients had received accumulated doses of less than 60 Gy. More than half of the cases were initiated by removal of teeth; however, one third occurred spontaneously. ORN developed within the first 3 years in 74%; but ORN can emerge on a traumatic basis for an infinite number of years after radiation therapy. Widespread ORN may be symptomless apart from a discrete dehiscense of the oral mucosa. The predilection site for ORN is the mandibular molar region. All cases of ORN but 1 were found in the field of radiation. CONCLUSION Because many ORN case are symptomless, more focus on mucosal dehiscence in the follow-up after radiation therapy is advocated in an effort to detect ORN at an early stage. The consequence of practically all ORN cases being located in the field of irradiation, together with tooth removal frequently being the initiating factor, should lead to a more aggressive preirradiation approach to dental pathology located within the field of radiation.