A case of malignant transformation of a benign chondroblastoma of the tibia in a fifteen-year-old boy is reported. The tumour was not irradiated at any time during the course of the disease. The tumour showed a typical appearance of benign chondroblastoma at the first operation. It recurred repeatedly after curettage, and finally emerged as a highly malignant undifferentiated sarcoma. In spite of amputation the patient died nine years after the onset of the disease with an enlarged liver and inguinal lymph nodes.