The hemolytic crises observed in patients with familial spherocytosis were initially thought always to represent a rapid increase in magnitude of the existing hemolytic process. It was first recognized by Lyngar1that, in some of these cases at least, the crisis was due to a sudden decrease in red cell delivery rather than to an increase in the rate of hemolysis. However, the significance of his observations was overlooked until Owren2reported a series of six crises, four of which occurred sequentially in members of a family with familial spherocytosis. In all of these patients severe anemia, thrombocytopenia, and reticulocytopenia were noted. The most significant observation, however, was bone marrow hypoplasia during the crisis, predominantly involving the erythroid elements. The myeloid and megakaryocytic elements showed only slight left shift, although granulocytopenia and thrombocytopenia were present at the time. This observation, plus the demonstration of a simultaneous decrease in