Although much progress has been made in the treatment of lymphomas, the unclear molecular etiology limits its further development. Interleukin-9 (IL-9) was initially described as a growth factor secreted by activated helper T cells type 2 (Th2). Various observations have demonstrated its diverse actions in immune and inflammatory responses. In recent years, a resurgence of interest in IL-9 has been spurred by the expanded identification of its cellular sources and biological targets. Also, the determination of its growth-proliferative and anti-apoptotic activities on multiple transformed cells implies a potential role of this cytokine in tumorigenesis. In this article we review the biologic properties and signal transduction pathways of IL-9, and furthermore discuss its possible role in lymphomagenesis as well as its impact on non-malignant infiltrating cells which are characteristic of the tumor microenvironment.