Asthma is a complex heritable inflammatory disorder of the airways in which the development of clinical disease depends on environmental exposure. It has been well established that T helper type 2 (T_H2) lymphocytes and their cytokines have an important role in allergic asthma. Interleukin (IL)-9, a member of the T_H2 cytokine family, has recently been implicated as an essential factor in determining mucosal immunity and susceptibility to atopic asthma. In this review we examine the critical experiments and observations that support this hypothesis. We also discuss these results in comparison with the experiments supporting the involvement of other T_H2 cytokines such as IL-4, IL-5 and IL-13.