It has long been known that T cells have the potential to modulate hematopoietic response in different ways. More recently, the importance of interleukin (IL)-17-secreting Th17 cells in T-cell-mediated regulation of hematopoiesis was indicated by the line of evidence that IL-17 links T-cell function and hematopoiesis through stimulation of granulopoiesis and neutrophil trafficking. Furthermore, our data demonstrated that IL-17 also affects other cells of hematopoietic system, such as erythroid progenitors, as well as mesenchymal stem cells. In order to better understand the regulatory role of IL-17 in hematopoiesis, molecular mechanisms underlying the effects of IL-17 on hematopoietic and mesenchymal stem cells were also studied.