Infections with gastrointestinal helminths elicit potent Th2 responses, which ultimately result in their expulsion. However, during expulsion of Trichinella spiralis this Th2 response also induces a severe enteropathy characterized by villus atrophy and crypt hyperplasia. Inducible costimulator (ICOS), a homologue of CD28, interacts with B7-related protein 1, and has been shown to be important in T-B cell interactions and antibody class switching. Significantly, ICOS appears to be involved in the induction of both Th1 and Th2 responses, but may be of heightened importance in Th2 responses. Here we employed a blocking antibody against ICOS to investigate the contribution of ICOS costimulation to the development of the protective and pathological immune responses induced during infection with T. spiralis. We show that, although blocking ICOS resulted in a decrease in TNF-α and the Th2 cytokines IL-4 and IL-5 and serum levels of total IgE, it did not affect the expulsion of the adult parasites. Surprisingly, levels of IL-9, IL-13 and IL-10 were elevated and protection against the larval muscle stage of the parasite was enhanced. Importantly, these findings may relate to the fact that ICOS blockade significantly ameliorated the enteropathy that usually accompanies expulsion of the adult parasite.