Aims: We analyze the possible clinical differences between bone jaw exposed areas in ONJ (osteonecrosis of the jaws) and ORN (osteoradionecrosis). Patients and method: Group 1 was composed with 53 ONJ cases and group 2 with 20 ORN cases. In both groups we analyzed, the major size of the exposed bone areas, the number of exposed areas, the location on the jaws and the presence of others associated and severe complications, such as skin fistulas and jaw fractures. We also investigated the possible local aetiology or trigger factor of the lesions. Results: The major size of the bone exposed areas was 2.29±2.02(mean ± std.dev) in group 1 and 2.7±2.9 (mean ± std.dev) in group 2 (p>0.05). The number of exposed areas was 1.8±1.34 (mean ± std.dev) in group 1 and 1.2±0.55 (mean ± std.dev) in group 2 (p>0.05). There were more fractures in the second group (20%) (p<0.05), and skin fistulas (35%) (p<0.05). We found more patients in group 1 in which the dental extraction was the local aetiology of the bone necrosis (35 cases, 66.03%), while in group 2 there were 8 (40%) (p<0.05). Conclusions: In our study with ONJ there were not differences in the major size of the bone exposed areas, but there were more lesions per patient than in group with ORN. The severity of the complications, such as jaw fractures and skin fistulas were higher in ORN, and in this group it was more frequent the spontaneous lesions than in the ONJ where it is more frequent following dental extractions.