The case of 52-year-old man is presented, who had suffered from pains in his right brachial region and in whom, upon admission to the Lower Silesia Centre of Oncology, a tumour of 20 cm in diameter and restricted mobility was disclosed in the right brachial region and proximal 1/3 of his right arm. Radiograms of his right humerus disclosed non-uniform restructuring of the osseous tissue, dominated by osteosclerosis in the upper half of his right humerus, while in the surrounding dift tissues of the proximal portion non-uniform shades were seen of calcified appearance. The patient was qualified to surgical biopsy and histopathological examination of the sample disclosed Chondrosarcoma G II. Following amputation of the right upper extremity together with the scapula, the tumour was subjected to histopathological and immunocytochemical examination in the Department of Pathomorphology, Lower Silesia Centre of Oncology. In numerous samples of the tumour dedifferentiated chondrosarcoma was diagnosed with a dominating component of malignant fibrous histiocytoma (MFH), which was confirmed by detecting a high expression of alpha-1-antichymotrypsin (ACT) within the spindle-shaped cell component of the chondrosarcoma. Spindle-shaped elements in dedifferentiated chondrosarcoma (DChSa) may represent patterns of fibrosarcoma, osteosarcoma or the malignant fibrohistiocytoma (MFH) type and they are present as if in the form of restricted fields with no reciprocal infiltrates. Moreover, due to the worse prognosis in the case of the MFH component in dedifferentiated chondrosarcoman as compared to that in classical chondrosarcoma, common evaluation of the material was made by a surgeon, radiologist and histopathologist, accompanied by a minimum panel of immunocytochemical tests (ACT, Vimentyna, Desmina, S-100). This enabled a final diagnosis for all tumour components to be established and prognosis to be made regarding the further fate of the patient.