The fascinating observation of Pearson and his coworkers, in this issue of the Journal, that traumatized spleen tissue may regrow (or be "reborn") into splenules after its spillage into the peritoneal cavity can probably be explained by the phenomenon of work-stimulated hypertrophy. That work does provoke spleen tissue to grow is suggested by the progressive splenomegaly that regularly accompanies chronic hemolytic disorders — particularly in patients in whom the spleen is the primary site of sequestration and destruction of abnormal red cells (e.g., in hereditary spherocytosis and the thalassemias). Conversely, splenic atrophy can be induced in some animals by . . .