Using a monoclonal antibody enzyme immunoassay, the concentration of interleukin-8 (IL-8) in cerebrospinal fluid (CSF) from 52 patients suspected of having meningitis was studied. The CSF IL-8 concentration was significantly higher in septic meningitis of known and unknown etiology than in aseptic meningitis and significantly higher in aseptic meningitis than in patients without meningitis. The CSF levels of IL-8 correlated with the levels of tumor necrosis factor-alpha, leukocyte count, neutrophil count, protein level, CSF/blood glucose ratio, and the number of days patients were hospitalized. The IL-8 values used to distinguish septic from aseptic meningitis, at a cut-off point of 3.00 micrograms/l, showed a sensitivity of 81%, a specificity of 92%, and a positive predictive value of 96%. The results suggest that determining IL-8 levels may be useful in the differential diagnosis of meningitis.