An ideal sepsis biomarker should be able to segregate infected patients from other causes of SIRS, and also to allow some kind of risk stratification. Furthermore, it should be capable of identifying subgroups of patients with specific sepsis complications, enabling target-specific preventive and therapeutic measures. Finally, access to this biomarker should not depend on complex and high-cost equipments and reagents, allowing access to more patients. New hematologic automated analyzers used for evaluation of the complete blood count provide a series of advanced analytical parameters that permit more detailed evaluations of circulating blood cells. Parameters such as the immature reticulocyte fraction (IRF) and immature platelet fraction (IPF) identify early signs of hematopoietic recovery, and have been studied in several inflammatory conditions. Recently, a study performed in critically ill patients suggested that the IPF could be a more accurate biomarker of sepsis development than C-reactive protein (CRP) and procalcitonin. The aim of this study was to evaluate whether IPF and IRF levels presented any association with clinical and laboratory parameters of sepsis severity.