Gliomatosis cerebri (GC) is a rare neoplasm in which there is a diffuse cerebral infiltration by malignant glial cells with relative conservation of the underlying structures. A 67-year-old lady was admitted complaining of balance problems, troubled breathing, stuttered speech, decreased mobility, progressive ataxia and also some mild cognitive problems. MRI demonstrated ill defined T2 hyperintensity with mild mass effect mainly involving the brain stem and cerebellar hemispheres, with minor signal abnormalities extending supratentorially along the corticospinal tracts. The imaging appearances were static over a year. No biopsy was performed. The patient received palliative care and died 2 years after initial presentation. Macroscopic examination of the brain showed an extensive firm white-grey lesion predominantly in the cerebellar white matter, the brainstem, spreading to the full length of the spinal cord and invading the sensory ganglia. Histology revealed an extensively infiltrating diffuse glioma with small elongated fusiform nuclei. Diagnosis of GC type 1 was made. Molecular genetic tests revealed BRAF V600E mutation, while no IDH1 & IDH2 mutations were found. GC should be taken into account in the differential diagnoses mainly when there is rapid clinical deterioration without clear evidence of radiological progression. Extensive spinal cord infiltration has been reported only in 9 % and BRAF V600E mutation was detected only in one case in GC previously. Future clinical trials should address whether BRAF V600E mutant brain tumour patients will benefit from BRAF V600E-directed targeted therapies.