IL-9 was first identified in the mouse system as a T cell growth factor while human IL-9 was isolated based on its activity on a human myeloid leukemic cell line. The high sequence homology between mouse and human IL-9 plus the preliminary biological analysis have predicted a wide spectrum of in vitro biological activities for this cytokine. Despite the high sequence homology between mouse and human IL-9, many biological activities, however, have not been demonstrated in both species. In vivo studies therefore become essential to understand the physiological role of IL-9. Increasing evidence have also suggested the possible role of IL-9 in the pathogenesis of HD and LCAL. Detailed analysis of IL-9 expression, autocrine growth and clinical outcome of HD patients will be required to make any speculation on the role of this cytokine in the disease states.