We report a case of a malignant peripheral nerve sheath tumor arising from a retroperitoneal ganglioneuroma in an adult male. There was no known previous history of a childhood neuroblastoma or of radiation therapy. In addition, the patient had no stigmata or family history of von Recklinghausen's neurofibromatosis. This case provides evidence that, although rare, spontaneous malignant transformation of ganglioneuroma can occur. Immunohistochemical studies supported that the spindle cell component was of nerve sheath origin.