BACKGROUND: Microcirculatory derangement in sepsis plays a crucial role in the impairment of tissue oxygenation that can lead to multi-organ failure and death. The change of RBC rheology in sepsis has been known to be important factors in microcirculatory derangement. Several studies have demonstrated that RBCs have decreased deformability in sepsis. We investigated the relationship between multi-organ failure and spherical index of RBC estimated by flow cytometer in critically ill patients with or without sepsis compared with the relationship in healthy volunteers. METHODS: Fourteen non-septic critically ill patients, 18 septic patients and 10 healthy volunteers were evaluated. We obtained peripheral venous blood from each patient and analyzed the change of RBC shape using flow cytometer (Becton Dickinson FACSCalibur) within 90 minute. The change of RBC shape was accessed with spherical index (M2/M1). A decrease in M2/M1 was correlated with the sphericity of the RBC and considered to have a lower capacity to alter their shape when placed in microcirculation. Multi-organ failure was accessed with sequential organ failure assessment (SOFA) score. RESULTS: The M2/M1 ratio of healthy volunteers, non-septic patients and septic patients were 2.25+/-0.08, 2.16+/-0.39 and 2.05+/-0.53, respectively. But, there was no significant difference between each group (p>0.05). And, there was no significant correlation between M2/M1 ratio of septic and non- septic patients and SOFA score (p>0.05, r2= -0.13). CONCLUSIONS: In our study, the spherical index of RBC was not associated with multi-organ failure in sepsis. But, further studies may be needed to evaluate the role of RBC rheology in sepsis.