b＞Objective/b＞br＞ We aimed to investigate the effects of hereditary hemolytic anemia (HHA), iron deficiency anemia (IDA), immune thrombocytic purpura (ITP) and aplastic anemia (AA) on hemorrheologic parameters, especially erythrocyte deformability. Erythrocyte deformability is expressed as the elongation index (EI) at a shear stress of 3 Pa. The EI of erythrocyte is defined as (L − W)/(L + W), where L and W are the major and minor axes of the ellipse, at a shear stress of 3 Pa. br＞br＞ b＞Materials and methods/b＞br＞ Thirteen HHA patients (age 15.3 ± 14.7), 16 IDA patients (age 9.6 ± 5.7), 16 ITP patients (age 4.8±5.4) and 6 aplastic anemia patients (age 15.3 ± 8.0) and 14 healthy controls (age 11.7 ± 12.7) were enrolled. Erythrocyte deformability were estimated by ectacytometer (RheoScan-D300; Rheomeditech, Seoul, Korea). Differences between HA, IDA, ITP, AA and healthy controls were compared. br＞br＞ b＞Result/b＞br＞ The EI value of IDA and AA were significantly decreased compared with normal control, but those of ITP and HHA were similar to normal control. But when shear stress were greater than 4 Pa, EI value of HHA were significantly decreased compared with normal control.br＞br＞ b＞Conclusion＞/b＞br＞ This study shows different RBC deformability in various hematologic disease. Diagnostic and prognostic significance of RBC deformability is needed.