BACKGROUND: Invasive meningococcal infections can be devastating. Substantial endotoxemia releases mature and immature neutrophils. Endothelial margination of mature neutrophils may increase the immature-to-total neutrophil ratio (ITR). These changes have not been previously well-described in invasive meningococcal disease. METHODS: Using 2001 to 2011 data from the US Multicenter Meningococcal Surveillance Study, the diagnostic sensitivity and clinical correlates of white blood cell count, absolute neutrophil count (ANC), immature neutrophil count (INC) and ITR were evaluated alone and in combination at the time of diagnosis of invasive meningococcal disease. RESULTS: Two hundred sixteen patients were evaluated: meningococcemia (65), meningitis (145) and other foci (6). ANC =1000/mm(3) or >/=10,000/mm(3) was present in 137 (63%), INC >/=500/mm(3) in 170 (79%) and ITR >/=0.20 in 139 (64%). One or more of these 3 criteria were met in 204 of the 216 (94%). Results were similar for meningococcemia and meningitis subgroups. All 13 cases with mildest disease met 1 or more of the 3 criteria. Eight children presented with ANCs <1000/mm(3): 3 of them died and a fourth required partial amputation in all 4 limbs. CONCLUSIONS: Invasive meningococcal disease is characterized by striking abnormalities in ANC, INC and/or ITR. Neutropenia was associated with a poor prognosis. Notably, without INCs, 37% of cases would have been missed. Automated methods not measuring immature white blood cells should be avoided when assessing febrile children. Serious infection should be considered when counts meet any of the 3 criteria.