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We present the case of an 8-year-old boy with pain in the distal part of the right thigh that occurred after a ball injury, without haematoma or oedema. The plain film showed exostosis of the distal part of the right femur. In another institution the "exostosis-like" formation was surgically removed and the histological finding was an aggressive type of chondroblastoma. A few months later, on the site of the surgical intervention, a significantly enlarged painful solid mass appeared. Radiography showed a large dense mass at the level of distal femoral metadiaphysis with tumour matrix mineralization, and Codman's sign. Computed tomography showed a large soft tissue mass, which arose from the surface of the underlying cortical bone and signs of high-grade malignancy. CT is a valuable method in predicting high-grade malignancy. Arteriography was also done. After operative treatment - seq. Campanacci, histology showed high-grade surface osteosarcoma of chondroblastic type. Pre- and post-operative chemotherapy was carried out. The boy died 2 years after diagnosis because of recurrent disease. High-grade surface osteosarcoma has the worst prognosis, compared with other surface osteosarcomas. Therefore, timely diagnosis is indispensable.