Leiomyosarcoma of bone is a rare tumor in an unusual location. Previous analysis of this entity mostly involved small numbers of cases with limited follow-up. Thirty-three patients with leiomyosarcoma of bone between 1977 and 1996 were studied, and the histologic appearance and grade were correlated with subsequent treatment and clinical behavior. To be included in this study the tumor had to be intraosseous, with other primary sites of origin clinically excluded. Also, most of the sarcomatous tissue (> or =70%) had to be of intramedullary location with only limited extraosseous extension. The patient's age at diagnosis ranged from 13 to 77 years (average 44.4). The gender distribution was equal. The long bones were preferentially affected (64%), with the lower extremity, around the knee joint, predominantly involved. Five patients (15%) developed postradiation leiomyosarcomas. The histologic analysis showed that the osseous leiomyosarcomas are most commonly of the classic type, followed by the epithelioid, myxoid, and pleomorphic variants. Immunoreactivity for smooth muscle markers (smooth muscle actin, common muscle actin, desmin) was positive in all tumors, and ultrastructural confirmation was obtained in 21% of cases. All sarcomas were histologically graded, which accurately reflected the subsequent prognosis. Seventy-five percent of the lesions were high-grade and the rest low-grade. The histologic grade of the tumors correlated with both the recurrence as well as the metastatic rates and together with the clinicopathologic stage of disease represented the cornerstone on which prudent therapy should be based.