Long term follow up of three patients with aneurysmal bone cysts has allowed us to draw attention to features which facilitate diagnosis and determine adequate treatment. The lesion occurs usually under the age of twenty with a slight female predominance. Treatment consists in complete local excision. Failing this, curettage should be carried out, and this gives good results. The effectiveness of radiotherapy cannot be assessed from our series, but in our opinion it may not be necessary in every case and should be applied judiciously, as postradiation sarcomas have been reported. Special attention must be paid to the tendency for spontaneous regression and cure after incomplete removal, particularly in recurrent cases, with a view to avoiding more aggressive treatment than is necessary. Multilocation (vertebra and distant bone), endothelial proliferation, and obliteration of the lumen of the small blood vessels support the view that this is a vascular lesion.