Interleukin-18 is a proinflammatory cytokine produced by a wide range of cells and is involved in the pathogenesis of several inflammatory diseases such as atopic asthma. It was recently demonstrated that IL-18 acts on T cells to induce airway inflammation and airway hyperresponsiveness. These observations strongly indicate that IL-18 stimulates Th1 cells to produce cytokines and chemokines responsible for the airway infiltration and inflammatory responsiveness. Moreover IL-18 activates mast cells and basophils playing the important role in atopy. Atopic asthma is characterized by eosinophilic airway inflammation, remodeling, mucus hypersecretion and high serum levels of IgE. Most current data suggest that asthma drives development of a Th2 lymphocyte-predominant immune response, which is associated with atopy and IgE mediated inflammation via pathways involving the production of proinflammatory cytokines such as IL-18. Here we discuss the functional role of IL-18 in activation of mast cells and basophils and pathogenesis of allergic diseases.