Introduction: The ossifying fibroma is a benign injury, asymptomatic, slow-growing compared with osteosarcoma, a painful destructive malignancy, with poor prognosis.

Case: Male patient aged 37, referred to the service in June 2008 to present injury right maxilla, with biopsy of July 2006, with a diagnosis of ossifying fibroma. Physical examination shows tumour-like mass of 2 × 2 cm in area of 13, 14 and 15 indurated, painless, covered by normal mucosa that is not cause of tooth mobility. BAL is performed complete resection of the lesion with diagnostic pathology report ossifying fibroma. Two years later comes to control, which refers to enlargement has noticed and is scheduled for excision of the lesion. BAG is performed in bloc resection of the tumour involving canine tooth and sinus infiltrates destroying bone contouring, is delimited area beyond the tumour to resect in bloc, which includes palatal and vestibular table by a safety margin. Pathology resulting in high grade chondroblastic osteosarcoma with positive margins. Surgical margin is extended and studied with the same diagnosis with tumour-free margins.

Conclusions: The origin or malignant tumours are rare in benign sites, in this case, after several years of tracing an ossifying fibroma, osteosarcoma appears, we discuss the possible malignant transformation of these benign tumours, or the appearance of other tumor, in this case, malignant apparition in site of a benign tumor, so it is essential to monitor and make diagnoses with histopathology.