A 30-year-old female case of gliomatosis cerebri, which showed remarkable improvement after radiation therapy is reported. The lesion had spread widely in the frontal and parietal lobes of both sides, causing diffuse swelling of the cerebral hemispheres. A specimen obtained by open biopsy revealed diffuse proliferation of astrocytic (GFAP positive) tumor cells, associated with little tissue destruction. After whole-brain radiation of 60 Gy, the lesion shrunk gradually but markedly. Hemiparesis and mild dementia, which the patient had experienced before treatment, subsided completely within 6 months, until she was able to restart her job as a dentist. Although gliomatosis cerebri is thought to have a poor prognosis, this case demonstrates the potential of radiotherapy for this disease. Factors which may lead to a better outcome are discussed.