About 50% of malignant peripheral nerve sheath tumors (MPNST) arise de novo. The remainder arise from neurofibromas in NF1. The life time risk of MPNST in NF1 patients is 8% to 13%. The greatest risk is in patients with multiple deep-seated symptomatic plexiform neurofibromas (30% risk) and in those with large rearrangements (microdeletions) in NF1 gene (16% to 26%). Malignant change is heralded by rapid enlargement or sudden onset of pain in a preexisting neurofibroma of long duration (latent period of 10 to 20 years). In addition, there may be referred pain, paresthesias, and motor weakness. This amputation specimen shows fusiform swelling of the forearm caused by a large MPNST. The patient previously had a large plexiform neurofibroma resected from this location; note the scar.