Expulsion of the gastrointestinal nematode Trichuris muris is mediated by a T helper (Th)2-type response, involving interleukin (IL)-4, IL-9 and IL-13. Here, we show that Th2 response-associated resistance is dependent on the presence of IL-1alpha and IL-1beta. When lymph node cells from naive IL-1alpha- or IL-1beta-deficient mice were subjected to Th2 polarization in vitro, they failed to polarize in the presence of IL-4 alone, but required the addition of exogenous IL-1alpha or IL-1beta. Furthermore, we demonstrate that both IL-1alpha- and IL-1beta-deficient mice are susceptible to chronic T. muris infection and that the inability to expel the worms is associated with a defect in the development of a Th2 response in the mesenteric lymph nodes. These results provide the first demonstration of the critical role of IL-1 in regulating Th2 responses during gastrointestinal nematode infection.