Gliomatosis cerebri, a rare condition, requires clinical, radiological and pathological correlation for diagnosis. Mental and personality changes are the most common presenting symptoms with or without focal neurological signs. The widespread nature of the disease is revealed by CT or MRI. The shape of the brain may be maintained and pathological gross examination may be unremarkable or show hypertrophy without evident tumour. Microscopic examination reveals infiltration of the brain by variably differentiated neoplastic glial cells. We present a patient with gliomatosis cerebri in whom we experienced difficulties with diagnosis. The literature is reviewed and the diagnostic features summarised.