Abstract.We performed analyses of electrolytes, amino acids, albumin, alpha2‐macroglobulin, gamma‐globulin and LDH in the lumbar cerebrospinal fluid of children undergoing treatment for acute lymphoblastic leukemia, non‐Hodgkin‐lymphoma or acute myeloid leukemia. At the time of diagnosis signs of a disturbance of the blood‐brain barrier were found in some patients. During induction treatment with L‐asparaginase a rise of glutamic acid and a decrease of glutamine occurred. This finding correlated with slowing of the EEG. Treatment with vincristine was associated with a slight drop of sodium and chloride concentration in serum, but not in the cerebrospinal fluid. Central nervous system prophylaxis with cranial irradiation, and to a lesser degree with intravenous medium‐dose methotrexate, gave rise to a further deterioration of the blood‐brain barrier function as indicated by an increase in albumin, alpha2‐macroglobulin and LDH levels. During radiotherapy the concentration of several amino acids rose, probably due to a disturbance of active carrier mechanisms. Patients with elevated albumin at the end of radiotherapy more often suffered an early leukemia relapse while still on treatment. No other clinical or electroencephalographic correlations of altered barrier functi