Children with Down's syndrome (DS) often have small and abnormal thymuses, with lymphocyte depletion, diminution of the cortex, and loss of corticomedullary demarcation--a picture resembling thymic involution. Besides this, they have markedly enlarged Hassall's corpuscles, some surrounded by a sheath of lymphocytes. Patients with DS are known to have increased numbers of respiratory infections; they also have a higher incidence of lymphatic leukemia than do individuals who do not have DS. Studies of cell-mediated (thymic-dependent) immunity demonstrate that children with DS have both diminished numbers of T cells as well as functional deficiency of these cells.