Recent genetic and immunohistochemical studies propose that the primary aneurysmal bone cyst is a tumour and not a reactive tumour-simulating lesion. Based on a familial case of aneurysmal bone cyst the authors contacted 135 patients with this disease. Sixty-eight females and 67 males (median age 14 years; range 2-73 years) were asked if other family members had bone lesions. One hundred and seven patients (79%) denied having other family members with lesions, 23 patients (17%) did not answer, and five patients (4%) gave evidence of other bone lesions in the family. These data indicate that a predisposing genetic defect could be part of a multifactorial pathogenesis in the development of some aneurysmal bone cysts.