Background. Current laboratory tests often cannot distinguish between bacterial and aseptic meningitis rapidly and accurately. The ability to make a prompt diagnosis has important implications for the management and outcome of children with meningitis. The observation that leukocytes aggregate in the cerebrospinal fluid (CSF) has been previously reported, and it has been advocated as a reliable method to distinguish the causes of meningitis in children. Objective. To investigate the utility of CSF leukocyte aggregation as a screening test to distinguish between bacterial and aseptic meningitis. Methods. We compared the clinical and laboratory indices of 109 prospectively enrolled patients with meningitis (67 bacterial, 23 viral, 19 undefined etiology) and evaluated the validity of the CSF leukocyte aggregation test. The predefined leukocyte aggregation scores (LAS) were compared among the types of meningitis, and correlations with other markers of inflammation were calculated. Results. The median LAS was significantly higher (P < 0.001) in the bacterial (32.1%; range, 0 to 84.1%) than in the viral (0%; range, 0 to 16.6%) or undefined (0%; range, 0 to 20.7%) groups. The optimal sensitivity of the leukocyte aggregation test, 98.5 to 92.5%, was demonstrated with LAS values of 0 to 3%. The corresponding specificity was 64.3 to 88.1%. The peripheral white blood cell (WBC) count, serum C-reactive protein, CSF WBC count, blood culture, CSF Gram stain and CSF culture were inferior to the LAS as screening tests when compared individually. The LAS was as effective as CSF protein, TNF-alpha, IL-1-beta, IL-6 and IL-8 to predict bacterial meningitis. In a logistic regression model that included routine laboratory tests, the best predictor of bacterial meningitis was the LAS (odds ratio, 1.6 to 3.7). Significant correlations were demonstrated between the LAS and CSF protein, CSF WBC count, IL-1-beta, IL-6 and IL-8. Duration of symptoms before diagnosis, pretreatment with antibiotics, HIV-1 infection status and CSF red blood cell count did not significantly alter the LAS. Conclusions. There is no single test to diagnose the etiology of meningitis in children promptly and accurately. The finding of leukocyte aggregation in CSF might be of value as a sensitive adjunctive screening tool for the timely diagnosis of bacterial meningitis, recognizing that it has low specificity and potential practical limitations.