Background and study aim:In several cases of meningitis ; routinely used diagnostic procedures are unable to identify the cause of this disease. The aim of the present study was to differentiate between acute bacterial and viral meningitis as it is very important matter. Patients and methods:Fifty patients who admitted to Fever Hospital were included in this study. They were classified into 2 groups according to CSF criteria, Group I (22 patients) with viral meningitis, Group II (18 patients) with acute bacterial meningitis. In addition 10 subjects were included as control group (GroupIII) .All subjects were subjected to clinical examination, routine laboratory investigation, lumbar Puncture, CSF analysis (bacteriological, cytological and chemical). Also, Nitric Oxide (NO) levels in serum and CSF were measured. Results : The study found that serum and CSF NO levels were more highly significantly elevated in patients with bacterial meningitis than in patients with viral meningitis and controls. While there is no significant difference between patients of viral meningitis and controls. The rise of NO levels was significantly associated with protein, WBC and C-reactive protein levels. Conclusion : Patients with bacterial meningitis had highly significant elevation in the serum and CSF levels of NO. Their levels were correlated with markers of inflammation as CRP, WBC and protein level of CSF. Serum NO examination is an easy, rapid and cheap test. It can be measured in acute meningitis without waiting the results of culture to determine the nature of infection either viral or bacterial.