AbstractGliosarcoma is a rare variant of glioblastoma characterized by a biphasic pattern of glial and mesenchymal differentiation. It is unclear whether mesenchymal differentiation in gliosarcomas is because of extensive genomic instability and/or to a mechanism similar to epithelial–mesenchymal transition (EMT). In the present study, we assessed 40 gliosarcomas for immunoreactivity of Slug, Twist, matrix metalloproteinase‐2 (MMP‐2) and matrix metalloproteinase‐9 (MMP‐9), which are involved in EMT in epithelial tumors. Nuclear Slug expression was observed in >50% of neoplastic cells in mesenchymal tumor areas of 33 (83%) gliosarcomas, but not in glial areas (P < 0.0001). Nuclear Twist expression was observed in >50% of neoplastic cells in mesenchymal tumor areas of 35 (88%) gliosarcomas, but glial tumor areas were largely negative except in four cases (P < 0.0001). Expression of MMP‐2 and MMP‐9 was also significantly more extensive in mesenchymal than in glial tumor areas. None of 20 ordinary glioblastomas showed Slug or Twist expression in >10% neoplastic cells. Thus, expression of Slug, Twist, MMP‐2 and MMP‐9 was characteristic of mesenchymal tumor areas of gliosarcomas, suggesting that mechanisms involved in the EMT in epithelial neoplasms may play roles in mesenchymal differentiation in gliosarcomas.