Neurofibromatosis is an infrequent genetic disorder that was first clinically described by von Recklinghausen and it was classified into 7 types by Riccardi. Type I neurofibromatosis is the most common type, and it is characterized by neurofibromas of variable size and multiple cafe-au-lait spots with minimal CNS lesion, if any. Malignant tumors develop in 2 to 13% of patients with type I neurofibromatosis, but they are rarely found in the retroperitoneum. We surgically resected huge retroperitoneal malignant peripheral nerve sheath tumors in 2 patients with sporadic type I neurofibromatosis. One patient survived 10 months and the other survived 3 months after operation. Retroperitoneal malignant peripheral nerve sheath tumor shows a poor prognosis even after surgical resection because of frequent recurrence and the lack of effective adjuvant therapy. Early diagnosis and development of new adjuvant therapy are required to effectively treat this malady.