Primary leiomyosarcoma of extragnathic bone is rare; fewer than 50 cases are found in the literature. We report on two patients, adult men, with tumors located on the long bones (close to the knee joint). Radiographically, the tumors were shown as purely destructive, osteolytic, and infiltrative lesions. The diagnosis was based on light microscopy, including immunohistochemistry and ultrastructural examination. The tumor cells were uniformly positive for vimentin and muscle actin, but not for desmin. In one case, the cells were also positive for alpha-smooth muscle actin. Electron microscopy showed definitive smooth muscle differentiation, including cytoplasmic filaments with focal densities. Both patients died of pulmonary metastases.2103-10