To evaluate prognostic factors affecting surgically treated osteoradionecrosis (ORN) of the mandible, 13 patients with mandibular bone necrosis caused by irradiation for head and neck cancer were examined clinicopathologically. The subjects were included 12 men and 1 woman, with a mean age of 65.3 years at surgery. The mean total dose of irradiation was 61.6Gy. The clinical stage according to Epstein et al.(1987) was II B in 1 patient, III A in 4, and III B in 8. The mandible was resected 10mm or more from the lesion. Delayed healing was found in two patients, and three had recurrence of necrosis of the remaining bone. Patients whose mandibular periosteum had been resected previously who received mandibular reconstruction had a poor outcome. Pathological changes, such as stenosis of the inferior alveolar artery, fibrosis of bone marrow, and low osteocyte density in cortical bone, were found in all resected mandibles. Among them, mandibles lacking sequestra and bone remodeling had poor viability. Therefore, the periosteal blood supply should be conserved and surgical interventions, particularly heat, must be minimized to avoid recurrence of ORN.