A 46-year-old woman presented with a 2-month history of right hemiparesis and disorientation. CT showed only slightly abnormal findings, while the T2-weighted MRI findings disclosed high intensity in both hemispheres. Two weeks before her death, a biopsy specimen showed monotonous increase of gemistocyte-like cells, mimicking reactive gliosis. However, autopsy revealed some different cytological features; i.e. nuclear atypia and higher cellularity, and the diagnosis of gliomatosis cerebri was made. It is suggested that establishing an antemortem diagnosis of gliomatosis cerebri remains difficult in some cases, even with MRI and biopsy. Careful examination of autopsy material may be necessary for a definitive diagnosis.