21 off-therapy ALL patients treated with chemotherapy plus cranial irradiation were assessed using the Wechsler Intelligence Scale for Children and compared with a group of 15 off-therapy solid tumor children treated with chemotherapy and extracranial irradiation. Children with leukemia achieved lower IQ scores than children with solid tumors. The ALL group was divided in two subgroups, by onset of disease and beginning of treatment: before (13 subjects) and after (8 subjects) 5 years of age. The results indicate that the disturbance of intellectual functioning is produced in ALL children irradiated after 5 years of age. In order to check on a possible delayed intellectual deterioration, all ALL children were also tested twice, with one year interval between the two tests. On the second assessment the scores for each subject were essentially the same as before. Further studies on a large number of long term survivors are being carried out in order to find out whether the deficit is transient or permanent.