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The histological diagnosis of malignant peripheral nerve sheath tumor (MPNST) is challenging because of the wide morphological spectrum and suboptimal performance of conventional immunohistochemical markers. MPNST arising primarily in the bone is exceptional, and its definitive diagnosis, particularly out of the neurofibromatosis type 1 (NF1) context, is even more problematic. Recurrent inactivation of EED or SUZ12 in a majority of MPNSTs results in a complete loss of trimethylated histone H3 at lysine 27 (H3K27me3) immunoreactivity, making it a highly specific biomarker of MPNSTs. In this article, we report a case of sporadic MPNST of the proximal femur that showed complete loss of H3K27me3. The patient was treated with limb-sparing surgery and postoperative radiotherapy. He developed multiple lung and bone metastases 4 months after surgery. Our case confirms the utility of H3K27me3 immunohistochemistry to yield a definitive diagnosis of sporadic MPNST in a rare primary site.