This study investigated the relationship between calculated immature granulocyte (IG) counts and the severity of sepsis. Coagulation parameters, fibrinolytic indices, bacterial isolation rates in blood cultures, and mortality were observed in 237 patients with suspected sepsis. The difference in leukocyte subfractions (delta neutrophil index; DN) identified by a cytochemical myeloperoxidase reaction and by a nuclear lobularity assay was determined with a blood cell analyzer (ADVIA 120, Siemens, Inc.). DN was strongly correlated with manual IG counts (r = 0.75, p <0.05). Patients with high DN (>40%) averaged 40% lower in platelet count, 26% prolongation of prothrombin time (PT), and 35% diminution of antithrombin III (AT III) activity, vs those with low DN (5-20%). Overt disseminated intravascular coagulation was more prevalent as the DN increased, and reached a peak in patients with DN >40%. DN values were closely associated with PT (r = 0.35, p <0.05), AT III activity (r = -0.36, p <0.05), and platelet count (r = -0.27, p <0.05). Positive blood culture rate averaged 3.5-fold higher in patients with DN >40% vs the subgroup with low DN of 5-10%. The mortality rate of patients with DN >40% markedly exceeded the mortality rate of patients with DN of 5-10% (79% vs 15%, p <0.05). Thus, DN has implications for the severity of sepsis and may be valuable to assess the prognosis of patients with suspected sepsis.