Sixteen cases of primary leiomyosarcoma of bone are described. The patients, 11 males and 5 females, ranged in age from 9 to 74 years. The annual incidence of this tumor in Sweden was calculated to be 0.09 cases per million. This figure was obtained by reviewing a Swedish series of spindle cell sarcomas of bone of which one quarter (11/44) were diagnosed by us as primary leiomyosarcoma. The diagnosis was based on light- and electron-microscopic examinations using the same criteria as for leiomyosarcoma of soft tissues. Thirteen tumors were located in a long bone of an extremity (nine close to the knee joint) and three in the central skeleton. Radiographically, all the tumors presented as a purely osteolytic lesion, and three patients had sustained a pathologic fracture. In four of six cases angiography suggested malignancy by revealing hypervascularity, irregular tortuous vessels, and diffuse contrast opacification. Contrast-enhanced computed tomography, performed in two cases, showed hypervascular areas within the tumors. Scintigraphy showed a marked increase in radionuclide uptake in all five cases studied. The clinical behavior indicates that primary leiomyosarcoma of bone is highly malignant. Eight patients had died of the tumor and, of the eight patients who were alive at follow-up, two had metastases, and one had been operated on three times for a cutaneous metastasis, which had recurred locally twice. The remaining five patients had been continuously free of disease for 6.5 to 12.3 years.