Gliomatosis cerebri is considered grade III astrocytoma because of the short survival period of patients with this tumor, while the tumor histologically consists of widespread low grade astrocytoma cells. The authors tried to clarify this discrepancy by applying genetic analysis of the tumor. A 29-year-old man originally presented with mild headache and showed diffuse high intensity areas in both hemispheres and in the cerebellum by T2-weighted magnetic resonance imaging (MRI) without gadolinium-dimeglumine (Gd)-enhancement in T1-weighted imaging. Histological diagnosis was gliomatosis cerebri with diffuse grade II astrocytoma. Seven months after temporary improvement following irradiation and chemotherapy, he developed progressive mental deterioration, and died in one year after the surgery. At this time T1-weighted imaging showed Gd-enhanced lesions with enlargement only of the cerebellar tumor. Genetic analysis demonstrated positive FGFR 1 and less FGFR 2 mRNA in the tumor tissue, and FGFR 1 mRNA was beta type dominant. These results indicated that the genetic features of this tumor are similar to those of glioblastoma multiforme concerning FGFR expression. The authors conclude that genetic investigation of the tumor tissue is required to predict the prognosis of gliomatosis cerebri patients, in addition to imaging and histological examinations.