BACKGROUND Periostitis Ossificans (PO) is a non-suppurative type of Osteomyelitis, commonly occurring in children and young adults, in mandible. The most common cause for PO is periapical infection of mandibular first molar. Radiographically PO is characterized by the presence of lamellae of newly formed periosteal bone outside the cortex, giving the characteristic appearance of "onion skin". CASE REPORTS Two male children 11 years of age reported to the Department of Oral Medicine with a painless and persistent bony hard swelling in the mandible, with a short duration (Figs 1, 5). Both the patients had grossly decayed mandibular permanent first molar tooth with periapical infection and buccal cortical plate expansion (Figs 2, 6). The radiographic study revealed different appearances, the Orthopantomograph of case I showed a single radiopaque lamella outside the lower cortical border, without altering original mandibular contour (Fig. 3) and in case II showed a newly formed bony enlargement on the outer aspect of the lower cortical border without altering the original mandibular contour (Fig. 7). Occlusal radiograph of both the patients showed two distinct radiopaque lamellae of periosteal bone outside the buccal cortex (Figs 4, 8). Kawai et al. classified PO of mandible into type I and type II, based on whether the original contour of mandible is preserved or not. Each type is further classified into two sub types (Table 1). In case I, the orthopantomographic appearance is characteristic of type I-1 (Fig. 3), but the appearance in occlusal radiograph is characteristic of type I-2 (Fig. 4). In case II, the appearances in both the radiographs are characteristic of type I-2 (Figs 7, 8). CONCLUSIONS Apart from the typical onion skin appearance, PO shows various other radiographic appearances. The radiographic appearance of Periostitis Ossificans may reflect the duration, progression and the mode of healing of the disease process. The radiographic classification of PO depends on the type of radiographs taken for evaluation.