A 65-year-old woman complained of a pedunculated, huge mass on the anterior chest associated with neurofibromatosis type 1. The clinical and histological data showed the possibility of a malignant peripheral nerve sheath tumor. On electron microscopy, we found the characteristic findings of a malignant peripheral nerve sheath tumor, such as the foci of en-tangled cytoplasmic processes, undifferentiated spindle cells having large pleomorphic nuclei with prominent nucleoli, distinct basal cell lamina, rudimentary cell junctions, intermediate filaments and clumps of glycogen in the tumor cells.