Dedifferentiated chondrosarcoma is a rare but highly malignant manifestation that can occasionally arise in patients with cartilage tumours. There remains uncertainty as to the best treatment for this condition and in particular whether chemotherapy may have a role in improving prognosis. Members of EMSOS were invited to contribute data on patients, tumours, treatment and outcomes of patients with dedifferentiated chondrosarcoma. Eight centres contributed data on 317 patients from 7 countries. The mean age was 59 (range 15 to 89) and the most common site was the femur (46%) followed by the pelvis (28%). 25% of patients presented with a pathological fracture and the most common high grade component identified was MFH. 23% had metastases at diagnosis and these patients had a median survival of 5 months. 30% of patients received chemotherapy, with 47% under 60 having chemotherapy compared with 10% over 60. One third of this group had neoadjuvant chemotherapy and the rest had adjuvant reatment. 88% had surgery with limb salvage in 80% of this group. The overall survival was 38% at 2 years and 24% at 5 years but in patients without metastases at diagnosis these figures were 44% and 28% respectively. Poor prognostic factors for survival were: metastases at diagnosis, amputation or no operation, local recurrence, age over 60 and pathological fracture at presentation. We were unable to identify any group in whom chemotherapy appeared to have a survival benefit. Dedifferentiated chondrosarcoma carries a dismal prognosis. Although 30% of patients received chemotherapy in this study we were not able to prove that it improved survival. Early diagnosis and complete surgical excision still offer the best prognosis for this condition