A 49-year-old right-handed woman was admitted for an 8-month history of unusual headache and transient diplopia. Clinical examination and brain CT scan were normal. Two months later, symptoms of raised intracranial pressure developed and a brain CT scan showed small lateral ventricles and sulci without any abnormal contrast enhancement or tumor mass. Brain MRI with T2-weighted spin echo sequences revealed a hyperintense signal in the right temporoparietal region, whereas only a slight enlargement of this region was noted on T1 spin echo. The patient deteriorated rapidly and died with uncontrollable raised cerebrospinal fluid pressure. The diagnosis of gliomatosis cerebri was made at necropsy. Gliomatosis cerebri is a rare intracranial neoplasm of neuroepithelial origin. Spread of this tumor is particularly fast in the white matter compared with the gray matter and nuclei. Clinical symptoms are not specific. The diagnosis can be suspected by MFU showing an isointense or hypointense signal in the deep white matter on T1-weighted images, and largely a hyperitense signal on T2-weighted sequences. The diagnosis is confirmed by stereotactic biopsy or necropsy. No curative treatment is currently available. Radiotherapy can delay the rapidly fatal outcome. Our case illustrates the possible onset of this rare disease by isolated cephalgia with normal early CT scan.