Osteosarcomas are primary malignant bone tumours in which mesenchymal cells produce osteoid. It is generally the most common malignant bone neoplasm, although lesions of the jaw are uncommon. Osteosarcoma of the jaw (JOS) presents a lower incidence of metastasis and a better prognosis than osteosarcoma of the long bones. However, patients with JOS can exhibit advanced tumours, mainly when early diagnosis is not performed. This article reports on a case of an advanced osteosarcoma of the maxilla. A 38-year-old woman was referred for evaluation and treatment of recurrent fibrous dysplasia of the facial bones. The patient related that she had been diagnosed with fibrous dysplasia four years earlier and, since the first diagnosis, she was submitted to four surgical interventions, all followed by recurrences. The main clinical findings were redness and swelling of the facial skin, upper lip ulceration, and hard palate swelling. Computed tomography showed a large hyperdense mass in right maxilla extending to right orbit and left maxillary sinus. An incisional biopsy was performed and microscopic examination showed areas of osteoid and chondroid formation surrounded by a cellular stroma. The diagnosis of osteosarcoma was established and the patient was recommended for oncologic treatment. Unfortunately, she died six months after the diagnosis due to uncontrollable local spread.