BACKGROUND The loss of structural and functional integrity of red blood cells (RBCs) during storage, collectively referred to as "storage lesion," has been implicated in reduced oxygen delivery after transfusion. RBCs are highly susceptible to oxidative damage from generation of reactive oxygen species by autoxidation of hemoglobin. Therefore, we examined whether increased oxidative stress (OS) in stored RBCs is associated with impaired cell membrane deformability before or after transfusion. STUDY DESIGN AND METHODS Thirty-four patients undergoing multilevel spine fusion surgery were enrolled. OS in RBCs was assessed by the presence of fluorescent heme degradation products and methemoglobin, which were measured with fluorimetric and spectrophotometric methods, respectively. Deformability and aggregation were determined by ektacytometry in stored RBCs, autologous salvaged RBCs, and posttransfusion blood samples. RESULTS OS in stored RBCs was significantly increased with longer storage (R = 0.54, p = 0.032) and significantly higher than that in fresh RBCs (9.1 ± 1.3 fluorescent arbitrary units vs. 7.7 ± 0.9 fluorescent arbitrary units, p < 0.001). Deformability decreased (R = -0.60, p = 0.009) with increasing storage duration. OS was elevated (p < 0.05) and deformability was decreased (p < 0.05) in postoperative blood from patients who had undergone moderate (≥4 RBC units) but not minimal or no transfusion. Neither the decrease in deformability of RBCs nor the aggregation changes were correlated with OS. CONCLUSIONS Although stored RBCs show signs of increased OS and loss of cell membrane deformability, these changes were not directly correlated and were only evident after moderate but not lower dose transfusion in postoperative surgical patients. These findings suggest that factors other than OS may contribute to impaired rheology with stored RBCs in the clinical setting.