Interleukin-31 (IL-31) is reported to be involved in Th2 cell-mediated diseases. However, the regulatory effect of IL-31 in the pathogenesis of nasal polyps (NPs) has not been understood. This study is aimed to determine whether IL-31 production is associated with Th2 cytokine levels and clinical severity in patients with NPs. Thirty patients with NPs and fifteen normal controls were included, and IL-31 production was determined by immunohistochemistry and enzyme-linked immunosorbent assay, respectively. The relationship between IL-31 expression and clinical severity was also evaluated. Besides, the effect of IL-31 on Th2 cytokine expression by polyp epithelial cells was investigated. We observed significantly enhanced IL-31 mRNA and protein level expression in NPs compared with normal control. IL-31+ cells were found to be positively correlated with clinical severity of NPs. Furthermore, we provided the direct evidence that IL-31 up-regulates Th2 cytokines expressed by polyp epithelial cells. Our results demonstrate that enhanced Th2 cytokine levels was correlated with IL-31 expression in NPs and provide a possible explanation for IL-31's regulatory role in the pathogenesis of NPs.