Neurofibromatosis type 1 (NF1) or von Recklinghausen's disease is a genetic disease generally characterized by café-au-lait spots and neurofibromas. Malignant tumors of the nervous system, such as malignant schwannomas, gliomas, or astrocytomas, have been well known to coexist with neurofibromatosis. However, occurrence of malignant tumors unrelated to the nervous system is rare. We report an unusual case of a 29-year-old NF1 female suffering from malignant peripheral nerve sheath tumor (MPNST) that eventually developed osteosarcoma in the proximal femur. Osteosarcoma is the most common high-grade malignant bone tumor in which the neoplastic cells produce osteoid. At 23 and 24 years old, she underwent excision of MPNST in the left posterior thigh. No osteosarcomatous portion was identified in these specimens. The patient underwent postoperative chemotherapy. At 29, left proximal thigh pain and swelling appeared. Computed tomography demonstrated cortical bone destruction in the left proximal femur where MPNST occurred. Magnetic resonance imaging revealed extraskeletal growth of the tumor. Bone scintigraphy demonstrated increased uptake in the left proximal femur. Hip disarticulation was performed. The removed tumor was composed of highly anaplastic cells. Lace-like irregular osteoid formation was observed among the tumor cells. MPNST component was totally absent. The tumor was diagnosed as osteoblastic type osteosarcoma. Two months after disarticulation the patient died of bilateral pulmonary metastasis. The correlation between the histogenesis of osteosarcoma and the genetic abnormality in NF1 patients has not been elucidated, but the finding of osteosarcomatous transformation in this case suggests the divergent cellular differentiation to mesenchymal malignant tumors of neuroectodermal tissue in NF1 patients.