When administered at pharmacological dosages, glucocorticosteroid hormones alter leukocyte kinetics, phagocytic cell function, cell-mediated immunity, and, to a lesser extent, humoral immunity. These properties are used to advantage in the treatment of immunologically mediated disease. Corticosteroids are used to suppress pathological immune responses associated with autoimmunity, inhibit rejection of allogenic tissues after organ transplantation, and to diminish inflammation associated with a wide variety of hypersensitivity disorders. The mechanisms by which corticosteroids relieve these conditions are still not completely understood but have recently become more comprehensible. By understanding how corticosteroids exert their effects, we can make better clinical decisions in the management of immune-mediated disease.