Four cases of primary leiomyosarcoma of bone are presented. The histology of this rare tumour has been studied with a panel of monoclonal antibodies to the intermediate filaments desmin and vimentin, and to other markers including smooth muscle actin, myosin, alpha 1-antitrypsin, alpha 1-antichymotrypsin, lysozyme, S-100 protein and cytokeratin. The tumour cells were uniformly positive for desmin, vimentin, and in one case for smooth muscle actin; all the other markers were negative. The findings have been compared with other spindle cell lesions of bone and with electron-microscopy of the tumours. Immunohistochemistry allows the histological diagnosis to be made without the need to resort to ultrastructural studies.