The case of a 78-year old man with a non adherent subcutaneous tumour in the right iliac fossa was presented. In about one year and a half it stretched on the third superior part of the fore right thigh and appeared under the shape of a hard plate free from the osseous tissue. Microscopically, besides some fibrosarcomatous proliferations or very slightly differentiated ones with osteoid structures, the predominance of the osteosarcomatous images was found. The problems of pathogenesis and diagnosis of extraskeletal osteosarcomas were discussed.