The authors reviewed 121 cases of aneurysmal cysts of the long bones treated at the Tumour Centre of the Rizzoli Institute of Orthopaedics of which 103 had been followed up for more than one year. A classification is proposed based on 5 morphological groups and 3 grades of aggression. All except 4 of these lesions were treated by curettage or resection (2 were treated by radiotherapy alone, and 2 only had biopsies). The overall results showed a recurrence rate of 13%, none of which were in the inactive group. There was no prognostically significant difference in the recurrence rate between the active and aggressive groups. In the morphological groups there were significant prognostic differences. The authors therefore advise a programme of treatment based on an assessment of the morphological type and aggressiveness of these tumours.