The deformability of erythrocytes in Waldenstrom's Macroglobulinemia (WM) patients are analysed by shear stress diffractometer in vitro. Erythrocytes obtained from blood samples of WM patients are subjected to well defined shear forces, varying from 0.3 to 60 Pa, and their corresponding elongation indexes generated by diffraction pattern of a laser beam are recorded. For comparison, equal number of samples from normal adult volunteers is also studied. The results show a shear dependent decrease in deformability index in WM patient samples. The decrease in deformability index of WM patients is significant at high shear forces (>12 Pa) when compared to those of normal samples whereas it is not significant at low shear forces. As the deformability of erythrocytes together with elevated plasma viscosity and enhanced erythrocyte aggregation may cause potential flow stagnation at microvasculature, these studies are clinically relevant.