Deformability and lipid peroxidation (LP) have been compared in erythrocytes of 45 chronic hemodialysis patients and 30 healthy subjects. The relative cell transit time (RCTT), correlating negatively to deformability, was measured using a St. George filtrometer. Thiobarbituric acid reactive material was measured and expressed as nanomoles of malondialdehyde (MDA) per gram of hemoglobin as a marker of LP. RCTT and MDA were found to be significantly higher not only before but also after HD compared with controls. Weak negative correlations were found between RCTT and the dosage of EPO as well as between RCTT and the daily amount of urine. These observations indicate the importance of residual renal function and the beneficial effect of EPO on erythrocyte deformability. The mean values of results suggest that HD does not affect the erythrocyte injury. The individual modifications of RCTT and MDA are also discussed.