The potential value of the nitric oxide (NO) level as a prognostic marker in human brain diseases is investigated. Cerebrospinal fluid (CSF) collected from neurological patients was examined for NO content using electron paramagnetic resonance (EPR) spectroscopy. In adult patients with meningitis, the level of NO was higher than that in other groups of brain disorders, such as brain traumas and brain tumors. Very high levels of NO in the CSF appeared to be correlated with a high incidence of fatal outcomes. In children with meningitis, it was possible to differentiate between viral and bacterial origin of the disease as evidenced by the EPR analysis of the CSF. The results indicated that NO levels in the CSF can be a useful prognostic marker in neurological diseases.