A longitudinal electroencephalographic study of 100 patients suffering from acute leukaemia is reported. An high incidence of EEG abnormalities indicates the central nervous system involvement with no clinical manifestations. The patients have been divided in two groups: A) with normal EEG and B) with pathological (cerebro-meningeal localisation) but many causes (metabolic, toxic or microcirculatory disturbances) can produce the abnormalities. Moreover therapy may cause CNS disturbances with EEG evidence. The most damaging seems to be the simultaneous association of radiation therapy and methotrexate.