['Diabetes mellitus is accompanied by microvascular complications leading to organ dysfunction, while sepsis is a major cause of morbidity and mortality in diabetics. We addressed the hypothesis that red blood cell (RBC) deformability may be additively compromised in septic diabetic patients, leading to a further impairment of microcirculation.', "Forty patients suffering from severe sepsis, 12 patients suffering from diabetes and 24 diabetic patients with severe sepsis were enrolled. A filtration method and a hemorheometer were used to measure the RBCs' index of rigidity (IR).", 'We observed no differences in severity, organ dysfunction and outcome between diabetic and non-diabetic septic patients. Mean SAPS II score was 23.5% vs 26.8% in non-diabetic and diabetic septic patients, respectively. The mortality in non-diabetic septic patients was 22.5% and in septic diabetics was 34.3%, while septic shock occurred in 15.0% and 20.8%, respectively. We detected higher IR (17.72+/-6.31) in septic diabetics than in patients with diabetes and no sepsis (12.26+/-2.28, p< or =0.001) and in patients with sepsis and no diabetes (13.9+/-2.86, p< or =0.01).'] Do severe sepsis and diabetes mellitus have additive effects on red blood cell deformability?