Leiomyosarcoma usually arises in the uterus, gastrointestinal tract, retroperitoneum, or soft tissue. Primary leiomyosarcoma of bone is rare. We encountered two patients with primary leiomyosarcoma of the femur; one was a 24-year-old woman and the other, a 41-year-old woman. Bone destruction observed on plain radiographs was minimal in both patients. Magnetic resonance imaging (MRI) showed low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, with these areas being much larger than the findings to be expected from the plain radiographs. Histological examination revealed spindle-cell sarcoma, with an interlacing pattern, acidophilic cytoplasm and blunt-ended or "cigar-shaped" nuclei, in both patients. In both patients, immunohistochemical examination showed positive staining for vimentin, desmin, and alpha-smooth muscle actin. Extensive examination of the gastrointestinal tract and uterus revealed no primary lesion. Therefore, the leiomyosarcoma in the femur was considered to be primary rather than metastatic. Histological examination, including immunohistochemistry, and the exclusion of an extraskeletal primary lesion, is necessary in diagnosing primary leiomyosarcoma of bone.