To clarify the MR features of supratentorial malignant astrocytic gliomas that did not exhibit contrast enhancement of MR imaging, six MR parameters were evaluated in two glioblastomas and six anaplastic astrocytomas. Moderate to marked edema, mass effect, and moderate tumor heterogeneity were noted in all cases. Hemorrhage was absent in all cases. Border definition and cyst formation or necrosis were not consistent among the tumors. Histological examination demonstrated significant vascular proliferation in two cases, and mild to moderate proliferation in six. Non-enhancing malignant astrocytic gliomas are a diagnostic challenge. Analyzing perifocal edema and heterogeneity of each mass probably provides clues for a proper diagnosis.