A malignant peripheral nerve sheath tumor (MPNST) is an extremely rare soft tissue tumor in the general population. On the other hand, there is a higher incidence of MPNST in patients with neurofibromatosis type I (von Recklinghausen's disease). The common sites are the extremities, trunk, head and neck. However, an intraabdominal or intrathoracic manifestation is uncommon. This paper reports two patients, a 31 year-old woman with multiple neurofibromatosis presenting as an intraabdominal malignant peripheral nerve sheath tumor, and a 33 year-old woman with an intrathoracic malignant peripheral nerve sheath tumor. The patients were treated with chemotherapy followed by radiotherapy. However, one patient died as a result of disease progression 21 months after the diagnosis and the other patient is currently being treated with radiotherapy.