Gliomatosis cerebri (GC) is a diffuse glial tumor of the brain which infiltrates more than two cerebral lobes; and generally seen in adults. Here we present a teenager boy who is diagnosed with gliomatosis cerebri. A 13-years old boy suffered from altered mental status and headache. Cranial edema and herniation was seen at cranial computerized tomography. Magnetic resonance image findings showed left dominance bilateral changes. Pathologic analysis of brain biopsy was relevant with gliomatosis cerebri. We would like to emphasize that gliomatosis cerebri should be considered in the differential diagnosis in patients who presented with abnormal neurological signs, and no clinical and laboratory evidence of infection, in the presence of diffuse brain involvement on the radiological imaging studies.