We analysed the incidence of a mandibular osteonecrosis (ON) in 189 patients irradiated for a cancer of the oral cavity, or of the oropharynx or epipharynx, with a total target dose of at least 60 Gy between 1980 and January 1994. Target doses per fraction were between 2.0 and 1.2 Gy, number of sessions per week was between 5 and 10, and total target doses were between 60 and 78.2 Gy. No instance of ON has been observed after target doses between 60 and 65 Gy. Cumulative incidence of an ON needing a mandibular resection was: 24.8% in patients treated with a mandibular dose per fraction between 2.00 and 2.22 Gy and a total mandibular dose between 66.00 and 79.70 Gy; 19.6% in patients treated with a mandibular dose per fraction between 1.80 and 1.90 Gy and a total mandibular dose between 69.00 and 75.60 Gy, 2.2% in patients treated with two fractions of 1.20 Gy per day for 5 days in the week with or without simultaneous application of cis-platinum and a total mandibular dose between 75.20 and 82.00 Gy. Dose per fraction, association of the tumour with bone, and the volume of the horizontal ramus of the mandibula irradiated with a high dose were observed to be significant risk factors.