OBJECTIVE: To determine the performance of established predictors of mortality in pediatric acute meningococcal disease (MD) in a contemporary population and to develop a simple predictive score that will not vary with observer. DESIGN: Prospective study for development set and mixed retrospective and prospective study for validation set. Setting and PATIENTS: A total of 227 patients with clinical meningococcal disease who were referred to three multidisciplinary pediatric intensive care units from 1993 to 1999. Early deaths before transfer to pediatric intensive care unit and deaths from cerebral herniation were included in the analysis. MEASUREMENTS AND MAIN RESULTS: The product of platelet and neutrophil counts at presentation (PN product) predicts mortality from meningococcal disease better than either count alone and at least as well as established severity scores. The Glasgow Meningococcal Septicaemia Prognostic Score and Malley scores performed poorly in these populations. The positive predictive value (PPV) for a Glasgow meningococcal septicemia prognostic score of >/=8/15 was 17.5% (16 of 91; 95% CI = 9%-25%), significantly lower than published estimates of 30%-74%, (p <.01). The PPV for death (or amputation) with a Malley score of 3/3 was 50% (12 of 24; 29%-71%), significantly lower than the published value of 100% (p <.001). The PN product appears to be a useful predictor. For a PN product of <40, PPV = 82% (9 of 11), specificity = 99% (195 of 197), and sensitivity = 73% (23 of 30). The performance of this score was greatest in younger children <5 yrs of age in whom clinical cerebral herniation was not seen as a cause of death (0 of 21 deaths at <5 yrs of age; 4 of 9 deaths at >/=5 yrs of age). CONCLUSION: Established scores significantly overestimate the occurrence of adverse outcomes in meningococcal disease. This may reflect improved resuscitation and outcome or variability in the application of these scores. The PN product achieves similar prediction to the scores currently in use and is independent of the observer. Factors that reflect the extent of the inflammatory response rather than the care before presentation are becoming increasingly important.