As nitric oxide (·NO) is hypothesised to play a role in the immunopathogenesis of neurological complications associated with inflammation, we compared levels of cerebrospinal fluid (CSF) and serum ·NO metabolites in 24 patients with HIV-1 infection, to those in 58 non-HIV infected patients with neurological disorders. Levels of ·NO metabolites were correlated with blood-brain-barrier dysfunction. CSF and serum nitrate and nitrite levels were measured by the nitrate reductase and Griess reaction methods. The ·NO metabolites, nitrate and nitrite, were raised in the CSF and serum of patients with AIDS and central nervous system complications, when compared to non-HIV infected patients with inflammatory and non- inflammatory neurological disorders (median nitrate and nitrite: CSF=18.3 μM vs. 11.1 μM vs. 7.0 μM, P<0.001, and serum=53.8 μM vs. 50.3 μM vs. 41.4 μM, P=0.04, respectively). CSF nitrate and nitrite levels correlated with the albumin quotient. This study supports the evidence that ·NO is a potential mediator of blood-brain-barrier breakdown in inflammatory diseases of the central nervous system.