Malignant transformation of fibrous dysplasia is very rare. The frequency is increased in polyostotic forms, in McCune-Albright and Mazabraud's syndromes and previously irradiated cases. Pain, which is rapidly becoming worse over a short period unrelated to trauma is the most alarming symptom. Early radiological features of sarcomatous transformation are moth-eaten or cystic areas of osteolysis, cortical destruction and gradual formation of a soft tissue mass. The prognosis is unfavorable as most of the cases are in an advanced stage in the time of diagnosis. We present an unusual case of unsuspected secondary osteosarcoma arising in a previously unirradiated, monostotic fibrous dysplasia. A 46-year-old woman was admitted with hip pain, which worsened after a minor trauma occurred 1 year ago. Plain graphies of left femur showed a well-delineated lesion with endosteal scalloping and areas having a ground-glass appearance. The MRI revealed minimal contrast enhancement but no heterogenous signal intensity, cortical destruction, periost reaction or accompanying soft tissue component was noted. The lesion was initially curetted. But being diagnosed as osteosarcoma histologically, classical osteosarcoma protocol pre and postoperative chemotherapy was applied. Resected femur showed areas of fibrous dysplasia admixed with osteosarcoma having fibroblastic, chondroblastic and osteoblastic areas that were focally invading the soft tissue. Tumor viability was estimated as 95%. The clinical course worsened rapidly after the operation. She did not respond to postoperative chemotherapy and lost with pulmonary metastases less than a years' time after the operation. The case is presented to increase awareness on the possibility of malignant transformation in an otherwise unsuspected fibrous dysplasia.