Of tumours arising in otherwise normal bones, fibrosarcoma is about one-third as common as osteosarcoma and may have a very slightly better prognosis. A comparison of the aetiology and behaviour of forty-nine fibrosarcomata and 152 osteosarcomata indicates several similar features. Fibrosarcoma lacks the characteristic peak incidence in adolescence of osteosarcoma, but the age and sex distributions of both tumour types in patients of middle life--twenty-five to sixty-five years--are remarkably similar, even in their frequency. With fibrosarcoma, perhaps, lung metastases are fewer and appear later, thus contributing to the slightly better survival, but there is some increase in the proportion of extra-pulmonary secondaries. As with osteosarcoma, patients with fibrosarcoma show some increase in the length of post-metastatic survival when metastases are of later appearance. For the whole series the five-year crude survival rate was 21 per cent, better results being recorded for patients with histologically well differentiated tumours (30 per cent) and for long bone tumours when the patient was metastasis-free initially and the tumour was treated by prompt ablation (40 per cent). These are probably the best results one may expect for osseous fibrosarcoma without recourse to adjuvant antimetastatic therapy. Complete control of the primary tumour is likewise mandatory, and can be assured only by complete surgical removal when this is technically feasible.