1. The case history of a haemophiliac in whom a large haematoma of the thigh was treated by amputation of the limb is described.

2. Examination of the available radiographs and of the histology led to the conclusion that the cyst was subperiosteal in origin.

3. Evidence is presented to support the hypothesis that haemophilic pseudotumours are always associated with subperiosteal haemorrhage, and that those haemophilic cysts which are confined to muscle have little or no effect on the adjacent bone.