Gliomatosis cerebri is a rare tumor of the central nervous system, and here we report two cases of this tumor. Case 1: A 41-year-old female was admitted to our department for evaluation of activity loss and mental changes. MR imaging revealed a high intensity symmetrical lesion in the bilateral frontal and temporal regions, thalamus and midbrain on FLAIR images. The patient had no neurological deficits and no abnormal findings in clinical laboratory data, including data for the cerebral spinal fluid. A specimen obtained by open biopsy revealed widespread infiltration of neuronal structures by small astrocytic cells, although without destruction of the neuronal structures. The patient was diagnosed with gliomatosis cerebri, and surgery and whole brain radiation at 44Gy were performed. The MRI lesion showed some shrinkage 20 months after surgery, and the KPS score was 90, the same as that before admission. Case 2: A 56-year-old male was admitted for numbness of the face and extremities. MR imaging revealed a high intensity lesion in the bilateral thalamus and a ringed enhanced lesion in the right thalamus. A specimen obtained by open biopsy revealed anaplastic oligodendroglioma, which was diagnosed as gliomatosis cerebri. Radiation at 54Gy, chemotherapy (ACNU, vincristine) and gamma-knife surgery were performed, and two months later MR imaging showed that the tumor (including the ringed enhanced lesion) had shrunk markedly. His KPS was 90 at 13 months after onset. These cases suggest that radiation therapy is effective for gliomatosis cerebri.