Osteoid osteomas are the commonest benign bone tumours. There is frequently a delay in diagnosis, in some cases of many months or years. The plain film appearances are not always immediately recognised and the characteristic bone sclerosis may be absent in sub-periosteal intra-articular lesions. Computed tomography (CT) is the most specific investigation, and in most cases pathological. confirmation is not required. Open surgical removal of the lesion has now been replaced by CT guided percutaneous radio-frequency thermal. ablation. (C) 2007 Elsevier Ltd. All rights reserved.