To investigate the hypothesis that acute lymphoblastic leukaemia (ALL) and its treatment disturb central nervous system nitric oxide metabolism, 11 patients were studied. Serial cerebrospinal fluid (CSF) samples were collected throughout treatment and the concentration of nitrate plus nitrite (NOx) was measured. Compared to an age-matched reference population, CSF NOx was significantly increased before treatment and rose further during the induction phase of treatment. Concentrations remained high during consolidation treatment, but fell during continuing treatment and normalised by the end of treatment. In conclusion, ALL and its treatment cause an increase in central nervous system nitric oxide production. reserved.