Management of Sepsis would greatly benefit from the incorporation of simple and informative new biomarkers in clinical practice. Ideally, a sepsis biomarker should segregate infected from non-infected patients, provide information about prognosis and organ-specific damage, and be accessible to most healthcare services. The immature platelet fraction (IPF) and immature reticulocyte fraction (IRF) are new analytical parameters of the complete blood count, that have been studied as biomarkers of several inflammatory conditions. Recently, a study performed in critically-ill patients suggested that IPF could be a more accurate sepsis biomarker than C-reactive protein (CRP) and procalcitonin. In this retrospective study we evaluated the performance of IPF and IRF as biomarkers of sepsis diagnosis and severity. 41 patients admitted to two intensive care units were evaluated, 12 of which with severe sepsis or septic shock, and 11 with non-complicated sepsis. Significantly higher IPF levels were observed in patients with severe sepsis/septic shock. IPF correlated with sepsis severity scores and presented the highest diagnostic accuracy for the presence of sepsis of all studied clinical and laboratory parameters. No significant differences were observed in IRF levels. Our results suggest that IPF levels could be used as a biomarker of sepsis diagnosis and severity.