One of the major determinants of erythrocyte survival is membrane deformability, and an important intrinsic parameter of membrane deformability is the shear elastic modulus (mu) with higher mu values corresponding to increased membrane rigidity. Using a micropipette technique, we determined the shear elastic modulus of erythrocytes from 21 patients with myelodysplastic syndromes (MDS). Ten thalassemic patients and 15 healthy subjects served as controls. The shear elastic modulus of MDS erythrocytes was very significantly increased in all the patients studied, reflecting the rigidity of the membrane; the value of mu was also significantly higher in comparison with thalassemic cells. These data point to a fundamental change in the mechanical properties of the erythrocyte membrane in MDS. Biochemical studies of the membrane composition are clearly needed.