Mild anaemia among patients with hyperthyroidism, reduction in intracranial zinc content of erythrocytes and anti- haemolysis of whole blood of patients with Graves, disease under laboratory Condition are the facts which indicate that although erythrocytes are devoid of receptors for thyroid hormones, they are affected by the excess of these hormones. In order to find out the reasons for such changes, osmotic fragility of the erythrocytes were investigated in peripheral blood of hyperthyroid patients ( n=40) using phosphate buffers ( pH=7.38) that contained different concentration s of sodium chloride. These patients had the clinical symptoms for at least two weeks. The results obtained in this study showed that the erythrocytes of these patients at concentration of 72 mmol/L of sodium chloride were 21.67℅ more fragile compared to that of healthy control group ( p<0.001) (n=20). The erythrocytes lysis could be due to reduction of plasma total lipids or excessive work load of these cells during thyrotoxicosis. Another possibility could be the impairment of intracellular electrolytes and other elements which are essential for the proper functioning of these cells. Finally, it may be concluded that the erythrocytes of hyperthyroid patients in this study were more fragile than that healthy controls.