Malignant peripheral nerve sheath tumor (MPNST) is rare, but its association with neurofibromatosis type 1 (NF1) is well known. A 56-year-old man with NF1 was referred to our hospital for investigation of dyspnea. Computed tomography showed a huge mass occupying the right thorax and invading the inferior vena cava. A salvage resection was performed to alleviate the developing hypoxemia. Histological examination confirmed an MPNST. The tumor regrew rapidly and the patient died 6 weeks after the resection. This case report reinforces the importance of monitoring patients with NF1 carefully because of their increased risk of the development of malignant neoplasms.