1 . A case of parosteal osteoma with histologically low-grade sarcomatous areas is described.

2. Arteriography revealed abnormal arteries, the histological appearances of which are described.

3. Vascular shunts indicative of low-grade malignancy were also seen.

4. Reasons are given for accepting the view that this lesion is a tumour, originally benign, but liable to the development of low-grade malignancy.