We report 4 cases of malignant peripheral nerve sheath tumors (MPNST) with neurofibromatosis type 1 (NF1). Mean age was 29.5. Two of them had a family history. Three of them were male. All of them had enlarging mass and pain in the background of neurofibromas. Locations were popliteal, thigh and forearm. The masses were greater than 5 cm in diameter in each case. In two cases the mass was showing continuity with a nerve. One patient had a nonossifying fibroma as well as a MPNST. Wide excision and radiotherapy were applied to three of the patients. One of them did not take any therapy after surgical resection. Two of the patients died of lung metastases after a mean period of 12.5 months. In a majority of NF1 patients MPNST emerges from a preexisting neurofibroma. The patients with NF1 are at greatest risk for developing sarcomas, so they should be followed closely.