The authors describe the observation of a primary giant-cell tumour of the sacrum which displayed on histological examination of its relapse a striking tissue component the structure of which corresponded to a chondroblastoma. Such a chondroblastomatoid appearance is one of the excep- tional components by which osteoclastomas surprise and make histological conclusions difficult. The overall and CT picture had a special bubble-like appearance of the relapsing focus which penetrated into the lesser pelvis. Despite its clear cut borders and repeatedly confirmed benign histological appearance the prognosis of this tumour is, similary as in the majority of relapsing osteoclastomas, very uncertain.

        Key words: osteoclastoma of the sacrum - spinal column, primary tumours - chondroblastomatoid