Cytokines, also referred to as interleukins, are the major orchestrators of host defence processes, and, as such, are involved in insults, repair and restoration of tissue homeostasis. This review summarises recent findings on and emerging models of the biological roles of the double-edged sword interleukin-13 (IL-13), which have been principally obtained from studies in mice that are deficient for IL-13, or its components. IL-13-mediated functions not only contribute to the susceptible phenotype in Leishmania major infection but also seem to play (...) a protective role in chronic leishmaniasis. Moreover, IL-13 plays a key protective role in the expulsion of helminths from the gut while also actively contributing to the pathology in schistosomiasis. In allergic asthma, IL-13 has also been found to be a key factor. Therapeutic administration of an IL-13 inhibitor in mice successfully prevents both the allergic disease phenotype and schistosoma egg-induced lung pathology. If this scenario holds true in humans, we soon may have an efficient drug for treatment of IL-13-mediated diseases. BioEssays 22:646–656, 2000. © 2000 John Wiley & Sons, Inc. (shrink)