OBJECTIVE. This study describes radiographic and MR imaging features of primary leiomyosarcoma of bone. SUBJECTS AND METHODS. Twelve patients (five men and seven women, 39-79 years old) who were treated at two oncology centers for primary leiomyosarcoma of bone involving the femurs, tibia, ilium, and inferior pubic ramus were studied. None of the patients had preexisting disease or disease elsewhere at the time of diagnosis. Pathologic diagnosis was obtained in all patients. RESULTS. Radiographs of all patients showed a matrix that was exclusively osteolytic. In long bones (seven patients), the tumor had an average length of 11 cm (range, 7-17 cm) and revealed an elongated configuration. In the pelvis (five patients), the average length of the tumor was 10 cm (range, 4-15 cm). MR imaging confirmed an intramedullary lesion in all patients, with extension into the Soft tissues in eight patients and no identifiable soft-tissue mass in the remaining four patients. Four of the five pelvic tumors had a prominent soft- tissue mass, whereas only four of the seven long bone lesions revealed a soft-tissue mass that was, in all instances, small. The tumor was hypointense on T1-weighted images and showed heterogeneous signal intensity on T2- weighted conventional and fast spin-echo sequences. We saw low signal intensity (short T2) in eight patients and homogeneous hyperintense signal intensity in one patient. In the remaining three patients, T2-weighted spin- echo sequences obtained with fat saturation showed high signal intensity (long T2) in the tumors. CONCLUSION. Primary leiomyosarcoma of bone is a rare tumor that on radiography reveals no matrix and on MR imaging reveals areas of T2 shortening in relation to fat on conventional and fast spin-echo sequences.