Hereditary Spherocytosis (HS) is a common cause of chronic hemolysis, particularly in people of northern European extraction. HS is due to an inherited defect in the red cell cytoskeleton. A morphological feature of this disorder is the presence of spherocytes on the peripheral blood smear. In the absence of significant spherocytosis, a useful diagnostic procedure is the incubated osmotic fragility test. In patients with HS, there is increased fragility (i.e. hemolysis) when erythrocytes are suspended in hypotonic salt solutions. This effect is magnified after incubation for 18 hours at 37°C. This test will also be positive in patients with other causes of spherocytosis, such as autoimmune hemolytic anemia, and in a few rare congenital nonspherocytic hemolytic anemias.