In neurofibromatosis 1 (NF1) patients, transformation to malignant peripheral nerve sheath tumor (MPNST) is only seen in plexiform neurofibromas. The lifetime risk of MPNST in NF1 patients is estimated to be 8% to 13%. Patients with multiple deep-seated plexiform neurofibromas (especially in the neck or extremities) and those with large deletions in NF1 gene are at a greater risk of malignant transformation. Malignant change is heralded by rapid enlargement or sudden onset of pain in a preexisting neurofibroma of long duration. The diagnosis of low-grade MPNST arising in a neurofibroma is made on light microscopy. Such cases show a continuum of histologic features ranging from 1) areas without increased cellularity or mitotic activity and only occasional atypical nuclei to 2) hypercellular foci with numerous atypical hyperchromatic nuclei to 3) overt foci of low-grade malignant peripheral nerve sheath tumor. Terminology: 1) Neurofibroma: Focal nuclear atypia is commonly seen in neurofibromas and, by itself, is not sufficient for the diagnosis of malignancy. Such cases should not be labeled Atypical Neurofibromas to avoid confusion. 2) Neurofibromas with Atypical Features: This term refers to neurofibromas with high cellularity, nuclear atypia and mitotic activity where the overall histologic features do not reach the diagnostic threshold of malignancy. The case shown here meets these criteria. Neurofibromas with only nuclear atypia are not included in this category. 3) Low-grade MPNST Arising in Neurofibroma: There is diffuse nuclear enlargement (at least three times the size of normal Schwann cell nucleus) and hyperchromasia, increased cellularity, and slightly increased mitotic activity.