A 41-year-old woman presented with an unusual case of benign astrocytoma with marked pleomorphism manifesting as consciousness disturbance due to intraventricular hemorrhage. Magnetic resonance imaging revealed a well-enhanced hypothalamic mass protruding into the third ventricle. Despite partial resection of the tumor without additional therapy, there have been no signs of tumor regrowth for 6 years. The histological findings revealed solid proliferation of tumor cells with marked pleomorphism, contrary to the benign clinical course. However, no mitosis, necrosis, or endothelial proliferation were found and the MIB-1 index was less than 0.5%. Immunohistochemical staining indicated the glial origin of the tumor. The tumor was similar to pleomorphic xanthoastrocytoma, but the histological findings were not exactly identical, indicating a new histological entity.