A 61-year-old man with neurofibromatosis type 1 presented to the outpatient surgical clinic with an enlarging mass in his neck. On physical examination, he was found to have extensive cutaneous neurofibromas and a large, firm, fixed mass in the right side of his neck, with leftward tracheal deviation. Magnetic resonance imaging showed a large, heterogeneous neck mass causing displacement of the vasculature and trachea (Panel A). During surgery, a mass measuring 11.5 cm by 6.5 cm by 6.0 cm was resected (Panel B); pathological examination revealed the mass to be a malignant peripheral nerve-sheath tumor. Malignant peripheral nerve-sheath tumors are aggressive soft-tissue sarcomas that may develop within preexisting plexiform neurofibromas in persons with neurofibromatosis type 1. The patient received adjuvant radiation and chemotherapy. Four months after completing treatment, he had no evidence of recurrent disease.