It has been known that regulatory T (Treg) cells and mast cells (MCs) are involved in tumor immunity regulation, but the exact roles and mechanisms of Treg cells and MCs in B-cell non-Hodgkin's lymphoma (NHL) are incompletely defined. In the present study, we found that the number of Foxp3+ Treg cells and CD117+ MCs increased in B-cell NHL patients. Concomitantly, a high level of interleukin (IL)-9 was observed in the sera from B-cell NHL patients. Neutralizing IL-9 significantly inhibited tumor growth in the lymphoma model of murine, and this process was associated with down-regulation of Treg cells and MCs. Furthermore, IL-9 was also demonstrated to induce expression of MC-related genes and proliferation of MCs from the bone marrow stem cells. Collectively, our results indicate that Treg cell and MCs are involved in immunosuppression in B-cell NHL, and IL-9 is a key mediator of Treg cells and MCs in that process. These findings provide novel insight for the pathogenesis and possible therapeutic strategy of B-cell NHL.