The course of hereditary spherocytosis (congenital or familial hemolytic anemia) is subject to major or minor exacerbations or crises. Pancytopenia, reticulocytopenia, and extreme spherocytosis characterize the major crises, during which hypersplenic effects appear to play a major role. These are characterized by the combination of (1) an unusual degree of hemolysis with (2) inhibitory effects upon maturation and delivery of bone marrow cells. At the height of the crisis, an extreme degree of maturation arrest in erythropoiesis is present. Splenectomy, which is often urgently necessary, results in a very rapid increase in all the cellular elements of the blood, confirming the phagocytic and inhibitory effects of the abnormal spleen. Following splenectomy, no further crises occur. The presence of successive cases in the same family suggest the possible role of infection as a precipitating or trigger agent in initiating an abnormal splenic mechanism leading to crisis. The cause of the hereditary spherocytosis is commented upon and evidence bearing upon an autopsecific mechanism is discussed.