The etiology and pathogenesis of respiratory epithelial adenomatoid hamartoma (REAH) remain poorly understood, although some reports have suggested that REAH features an inflammatory process. T-helper type 9 (Th9) cells are a newly identified subset of CD4 T-helper cells characterized by the expression of high levels of interleukin (IL)-9, which may promote inflammation. As REAH may involve an inflammatory process, we evaluated whether IL-9 and/or Th9 cells were present in REAH and compared the levels thereof to those of normal nasal mucosa. Eleven patients with REAH and 5 exhibiting cerebrospinal fluid leakage were included in the study. Flow cytometry was used to measure Th9 cell numbers, a cytometric bead assay was applied to measure IL-9 levels, and real-time polymerase chain reaction was used to quantify the levels of mRNA encoding IL-9. Th9 cells, IL-9 mRNA, and IL-9 were detected in all REAH and control samples. The proportion of Th9 cells in the patients with REAH was significantly greater than that in the controls. The expression levels of IL-9-encoding mRNA and IL-9 protein were significantly higher in the patients with REAH than in the controls. The Th9 cell subset was expanded, the synthesis of IL-9-encoding mRNA was upregulated, and IL-9 secretion was increased in REAH tissue, suggesting that Th9 cells play a central role in the pathogenesis of the disease.