BACKGROUND: Irradiation of cellular blood products is the current method used for the prevention of transfusion-associated graft-versus-host disease. However, irradiation has been shown to cause biochemical changes in stored red blood cells (RBCs) and to generate reactive oxygen species (ROS). Irradiation-induced biochemical changes and oxidation damage of the RBCs is closely related to the deformability of these cells. Furthermore, deformation of the RBCs may lead to alterations in the post-transfusion viability of stored RBCs. In this study, we evaluated the physical properties of irradiated RBCs. METHODS: Citrate phosphate dextrose adenine-1 preserved RBC concentrates were irradiated with a minimum dose of 25 Gy and stored at 4oC for 28 days. The deformation of the RBCs was measured by a microfluidic ektacytometer (RheoScan-D). To examine the effect of oxidative stress, the RBC concentrates were exposed to oxidative stress using FeSO4 and ascorbic acid. RESULTS: The deformability threshold of the irradiated RBCs was significantly lower than that of the control RBCs on day 21 of storage and later (P<0.05). The deformability threshold of the RBCs exposed to oxidative stress was lower than that of control RBCs, and the difference was significant on day 21 of storage and later (P<0.05). For up to 21 days of storage, the deformability of control and irradiated RBCs were maintained; however, they were significantly decreased at 28 days of storage compared with 1 day of storage (P<0.05). CONCLUSION: Irradiation reduced the threshold of RBC deformability during storage. In accordance with the effect of irradiation, oxidative stress affected the RBC deformability. Therefore, a ROS scavenger may play a protective role against deformation of irradiated RBC concentrates. Further evaluation will be required for confirmation and clarification of these findings.