Background IL-9 is an important stimulus for tissue infiltration by mast cells, a feature requiring concomitant activation of c-Kit. Objectives We assessed IL-9 expression and c-Kit + mast cells in the nasal mucosa of patients with allergic rhinitis during seasonal pollen exposure and observed the effects of allergen immunotherapy. Methods We studied 44 patients with seasonal rhinitis and asthma before and 2 years after a double-blind trial of grass pollen immunotherapy. Nasal mucosal IL-9 + cells and c-Kit + mast cells were assessed by means of immunochemistry. Cell types expressing IL-9 protein were determined by means of dual immunofluorescence. IL-9 mRNA-positive cells were assessed by means of in situ hybridization, and their phenotype was determined by using sequential immunohistochemistry and in situ hybridization. Results Nasal mucosal c-Kit + mast cells were increased during the pollen season ( P = .0001). IL-9 mRNA-positive cells also tended to increase ( P = .1) and correlated with nasal EG2 + eosinophils ( r = 0.47, P = .05) and IL-5 mRNA-positive cells ( r = 0.54, P = .02). The cell sources of IL-9 included T cells, eosinophils, neutrophils, and mast cells. When compared with placebo, successful pollen immunotherapy markedly inhibited seasonal increases in nasal mucosal c-Kit + mast cells ( P = .001) and the seasonal expression of IL-9 mRNA-positive cells ( P = .06). Immunotherapy also inhibited IL-9 protein expression from nonendothelial cell sources ( P = .0007). Conclusion IL-9 is upregulated in the nasal mucosa during the pollen season and correlates with tissue infiltration by eosinophils. Successful pollen immunotherapy is associated with inhibition of seasonal increases in both nasal c-Kit + mast cells and eosinophils. This effect might be explained, at least in part, by the reduced local expression of IL-9.