1. The clinical, radiological, operative and histological features of twelve aneurysmal bone cysts are recorded.

2. Attention is drawn to atypical features, and the problem of diagnosis is discussed.

3. It is recommended that treatment should be complete excision, except where this would interfere with function.

4. Various theories on pathogenesis are considered and favour given to the view that these cysts result from a local change in haemodynamics occurring possibly in a pre-existing lesion.