55 children suffering from acute lymphoblastic leukemia were treated wih prednisone, vincristin-sulfate, daunorubicine and L-asparaginase (1st month), 6-mercapto-purine, cytosine-arabinoside. cyclophospamide, methotrexate-i.th. and cranial irradiation (2nd month). Maintenance-therapy comprised 6-mercapto-purine, cyclophosphamide and methotrexate-i.v. Serial EEG-examination were done in most of them during the course of treatment. 4 children presented with meningeal leukemia at the time of diagnosis. Their EEGs showed moderate slowing before treatment and at the end of the first month. One of them suffered a focal convulsion two days after the last VCR/daunorubine-injection. All EEG-changes were completely reversible later on. 3 patients showed severe focal EEG-disturbances and convulsions during serious neurological complications (intracranial bleeding, rubella-encephalitis). The remaining patients were free of complications at the time of the EEG-examination. Before therapy 24% of them presented normal findings, 57% slightly and 19% moderately abnormal EEGs. At the end of the first month there was an increase of moderate and severe EEG-slowing to 37% and 23%, only 11% of findings were normal. Improvement during the second month of therapy led to 37% normal and 57% slightly abnormal EEG-findings at its end. After maintenance-therapy of 1/2--1 year duration there were 65% normal EEGs and no moderate and severe disturbances any longer. Quantitative spectral analysis of the EEG in 6 additional children revealed similar changes.