Nine children treated for acute leukemia or lymphosarcoma developed subacute encephalopathy starting with listlessness, depression and impairment of speech. Walking difficulties, ataxia, spasticity and sphincter disorders developed later. Transient intracranial hypertension and abnormal movements respectively developed in two patients. EEG frontal slow waves, raised CSF protein, abnormal white matter radioisotope uptake and CT scan hypodensity with patchy contrast enhancement were evident at the onset. Later, dilated ventricles and calcification appeared in the younger patients. Post-mortem neuropathological studies of three patients disclosed predominantly perivascular myelin loss in areas of white matter necrosis, abnormalities of small vessels and numerous axonal swellings. The spinal cord showed secondary degeneration of the corticospinal tracts. Analysis of the aetiological factors in this series points to the prevailing danger of cranial radiotherapy, probably increased by the young age of patients and by associated drug administration.