Interleukin (IL)-18 was originally regarded to induce T helper cell (Th)1-related cytokines. In general, factors favoring interferon (IFN)-gamma production are believed to abolish allergic diseases. Thus, we tested the role of IL-18 in regulation of bronchial asthma. To avoid a background response of host-derived T cells, we administered memory type Th1 or Th2 cells into unsensitized mice and examined their role in induction of bronchial asthma. Administration of antigen (Ag) induced both airway inflammation and airway hyperresponsiveness (AHR) in mice receiving memory Th2 cells. In contrast, the same treatment induced only airway inflammation but not AHR in mice receiving memory Th1 cells. However, these mice developed striking AHR when they were coadministered with IL-18. Furthermore, mice having received IFN-gamma-expressing Th1 cells sorted from polarized Th1 cells developed severe airway inflammation and AHR after intranasal administration of Ag and IL-18. Thus, Th1 cells become harmful when they are stimulated with Ag and IL-18. Newly polarized Th1 cells and IFN-gamma-expressing Th1 cells, both of which express IL-18 receptor alpha chain strongly, produce IFN-gamma, IL-9, IL-13, granulocyte/macrophage colony-stimulating factor, tumor necrosis factor alpha, regulated on activation, normal T cell expressed and secreted, and macrophage inflammatory protein 1alpha upon stimulation with Ag, IL-2, and IL-18 in vitro. Thus, Ag and IL-18 stimulate memory Th1 cells to induce severe airway inflammation and AHR in the naive host.