ABSTRACTInterleukin (IL)-13 is a Th-2 cytokine, known to be involved in allergic diseases. Although IL-13 and another Th- 2 cytokine, IL-4, share receptors and signal pathways, having similar biological properties, the critical role of IL- 13 alone in vivo in the pathogenesis of bronchial asthma has recently been recognized, based mainly on analyses of mouse models. IL-13 and its signal pathway are thought to be promising targets to develop a therapeutic agent for bronchial asthma. In this article, we summarize the signal transduction pathway of IL-13, the pathological roles of IL-13 in bronchial asthma, and possible therapeutic strategies to block the IL-13 signal.