We have retrospectively studied 73 patients with mandibular osteoradionecrosis (ORN) following radiotherapy (high voltage external beam radiation alone, or combined with brachytherapy) ranging from 50 to 130 Gy (mean 76) with fields including the mandible. Based on this analysis we defined four distinctive stages: Stage 0: mucosal defects only; Stage I: radiological evidence of necrotic bone with intact mucosa; Stage II: positive radiographic findings with denuded bone intra-orally; Stage III: clinically exposed radionecrotic bone, verified by imaging techniques, along with skin fistulas and infection. The clinical features and characteristics of these stages are described, along with their clinical courses and outcome. The incompleteness of current ORN definitions, mainly emphasizing mucosal defects and bone exposure, and the need for a revised ORN classification is pointed out. Based on these observations and the insufficiency of previous definitions, we arrived at the following ORN definition: Radiological evidence of bone necrosis within the radiation field, where tumour recurrence has been excluded.