Interleukin-13 (IL-13) is a Th2-type cytokine, secreted from CD4(+) T cells, mast cells, basophils and eosinophils. It is an immunoregulatory cytokine. It has become evident that IL-13 is a key mediator in the pathogenesis of allergic inflammation. The important role of IL-13 in the pathogenesis of bronchial asthma as well as other allergic diseases has been recognized. Additionally, IL-13 plays a major role in various other inflammatory diseases including cancer.