95 cerebrospinal fluid samples from leukemic children, 50 with and 45 without meningeal relapse, were studied for glucose, total protein, protein electrophoresis, immunoglobulins, lactic dehydrogenase and its isoenzymes, muramidase and leukocyte count, in addition to blast cell investigation. Only total protein and albumin were elevated in children with CNS leukemia, this increase persisted for at least 2 weeks after intrathecal treatment began and was independent of presence of blast cells. Our findings also indicate that protein elevation has no value as prognostic factor for future CNS infiltrations.