To investigate the role of mast cells and interleukin-9 (IL-9) in B-cell non-Hodgkin lymphoma (B-NHL) development and its clinical significance.The expression level of CD117 in tumor tissues of 32 B-NHL patients was determined by Western blot. The infiltration of CD117⁺ mast cells (MCs) in human B-NHL tumor tissues was observed by immunohistochemistry staining. To evaluate the correlations between the data from CD117⁺ MCs and biological markers of human B-NHL, a Spearman correlation coefficient (rs) was calculated. IL-9 levels in sera of B-NHL patients were measured by ELISA. Effects of IL-9 on expressions of functional genes of mouse bone marrow-derived mast cells (BMMCs) were detected by real-time quantitative RT-PCR.The expression of CD117 was upregulated significantly in human B-cell NHL involved tissues when compared with that of controls (0.0551±0.0064 vs 0.0192±0.0072, P<0.01). Infiltration of more CD117⁺ MCs was found in tissues from B-cell NHL subjects compared with that of controls. IL-9 level in serum samples from patients with B-cell NHL was higher than that from healthy controls. Addition of rIL-9 to the culture gave rise to increase in the purity of mouse BMMCs in the first three weeks. In vitro culture experiments showed that the addition of IL-9 could induce the differentiation of mouse BMMC and the expressions of MC-related genes, including CD117, Fcer1α, Mcpt1 and Mcpt5.Our study showed that IL-9 promoted immune response mediated by MCs, and probably played important roles in B-NHL growth. Pharmacological or targeted inhibition of mast cells or IL-9 activity may provide new strategy for B-cell NHL therapy.