T helper (Th) 2 cytokines, particularly interleukin 4 (IL-4), IL-5 and IL-13, might be important in the development of allergic asthma. Humanized monoclonal antibodies (hMAbs) against IL-5, and a recombinant soluble human IL-4 receptor have been developed as possible treatments for this disorder. However, these approaches have not yet proven to be successful in the treatment of persistent asthma, suggesting that neither IL-4 nor IL-5 is important in asthma pathogenesis. Indeed, there is insufficient information about the efficacy of soluble IL-4 receptor and the anti-IL-5 hMAbs in the treatment of asthma to draw firm conclusions about the importance of these Th2 cytokines. Nevertheless, because IL-4 is required for IgE production and IL-5 is required for eosinophilopoesis, these Th2 cytokines must remain important candidates for a role in the pathogenesis of allergic asthma.