OBJECTIVE To investigate the relationship between red blood cell (RBC) shape and modifications of RBC membrane protein content in critically ill patients with or without sepsis compared with healthy control volunteers. METHODS Prospective, observational in vitro study. METHODS University-affiliated cell biology laboratory. METHODS Human erythrocytes from healthy volunteers and nonseptic and septic intensive care unit patients. METHODS Sialic acid membrane content was measured on isolated RBC membrane proteins by high-performance liquid chromatography. RBC shape, estimated by the spherical index (M2/M1) or by the moment and effect of osmolality on RBC shape, was studied by flow cytometry at 25 degrees C. Glycophorin A content was measured with antiglycophorin antibodies in flow cytometry. RESULTS Sialic acid content was lower in the septic than in the nonseptic patients (1.98 +/- 0.79, 2.20 +/- 0.39 microg/100 microg membrane protein, respectively; p =.01) and than in the volunteers (2.71 +/- 1.00 microg/100 microg membrane protein; p <.001). No significant difference was found in glycophorin A content between septic and nonseptic patients. RBCs from septic patients had a more spherical shape in isotonic solution than those of healthy volunteers, as assessed by a computed spherical index (M2/M1 ratio: 1.68 +/- 0.34 vs. 1.95 +/- 0.32; p =.001). Only the RBCs of septic patients failed to change their shape in hypo-osmolar solution (M2/M1 ratio: 1.68 +/- 0.34 in iso-osmolar, 1.56 +/- 0.28 in hypo-osmolar solution; p =.17). There was a significant correlation between the RBC shape evaluated by the spherical index or by the moment of the cytometric histogram and the sialic acid membrane content in all critically ill patients (septic and nonseptic patients) (r2 =.16, p =.01 for the moment, and r2 =.17, p =.01 for the spherical index, respectively). CONCLUSIONS RBCs of septic patients are characterized by a more spherical shape, a decreased capacity of sphericity in hypo-osmolar solution, and a reduction in the sialic acid content of the RBC membrane. These modifications in RBC shape and membrane may contribute to the RBC rheologic abnormalities frequently described in sepsis.