Hyperbaric oxygen (HBO) has been used as a tool in the management of osteoradionecrosis (ORN). However, it has not been conclusively proven to be of benefit. The precise role and guidelines for its use also have not been clearly defined. This report retrospectively analyzes 41 patients treated at the Hyperbaric Chamber Unit at the Toronto Hospital (Toronto General Division) with proven mandibular ORN from 1980 to 1985. The results show that 83% of the patients had a significant improvement with HBO therapy, judged by at least a 50% decrease in the size of the exposed bone, closing of the fistulous tract, or complete relief of symptoms. Within the group of patients who were significantly improved, 15% showed complete resolution of ORN. Seven (17%) of the patients did not benefit from the HBO. All seven patients had radiological evidence of dead bone. Based on these observations, the following conclusions can be made: 1. HBO is of benefit in the management of ORN. 2. Patients with ORN may be divided into two groups: mild and severe. 3. Cases of mild ORN will heal with HBO alone, but, in severe ORN, surgery is necessary to remove dead bone. 4. All patients with ORN should receive dental evaluation, local wound care, and a strict oral hygiene regimen. Diseased teeth should be removed prior to radiotherapy. Subsequently, any teeth that became abscessed should be extracted in conjunction with prophylactic HBO. Antibiotics play an ancillary role in the management of ORN. For patients who have received radiation to the mandible, the authors propose regular follow-up in order to detect ORN at a time when HBO can arrest the disease.