The erythrocyte fragility test, also known as the erythrocyte osmotic fragility test, refers to the fact that red blood cells in hypotonic sodium chloride solution gradually increase in size until they finally rupture and hemolysis due to the continuous absorption of water by the red blood cells. The erythrocyte osmotic fragility test provides useful information for the diagnosis of disease by understanding the resistance of erythrocytes to hypotonic solutions. An increased erythrocyte osmotic fragility test is commonly seen in hereditary spherocytosis, warm antibody autoimmune hemolytic anemia, and also in hereditary elliptocytosis, where the erythrocyte fragility test may be significantly increased. A decreased erythrocyte fragility test is commonly seen in thalassemia, but also in iron deficiency anemia, cirrhosis, and obstructive jaundice. Therefore, the erythrocyte fragility test can provide useful information about the type of disease to which the red blood cells belong.