Methotrexate and Aracytin commonly employed in prophylaxis and treatment of leukaemic neuromeningosis, can induce neuromeningeal injury. Cerebrospinal fluid (CSF) protein disturbance may reflect the damage. In this study we have examined 141 CSF samples collected by lumbar puncture from 51 subjects: 15 normal controls and 36 leukaemic patients during the prophylaxis or the Central Nervous System (CNS) involvement treatment. In all the CSF samples, total proteins, IgG and IgA were examined. CSF total protein and albumin levels increased in the presence of leukaemic neuromeningosis and also after a prolonged CNS prophylaxis or therapy. The same behaviour was found in IgG levels. The IgA levels did not show any modification. We conclude that these parameters are not reliable indices of leukaemic CNS involvement.