Rationale: Eosinophilic chronic rhinosinusitis (CRS) with nasal polyps (CRSwNP) represents a hard-to-treat subtype of CRS. Objectives: To determine the pattern of expression and biologic role of microRNAs (miRNAs) in CRS, particularly in eosinophilic CRSwNP. Methods: Global miRNA expression in sinonasal mucosa from controls, CRS without nasal polyps (CRSsNP), and patients with eosinophilic CRSwNP was compared using miRNA microarrays. MiR-125b expression was detected by means of quantitative reverse-transcriptase polymerase chain reaction. The cellular localization of miR-125b was determined by in situ hybridization. MiR-125b functional assays were performed on airway epithelial cells and mice. MiR-125b expression regulation was studied by tissue and cell culture. Measurements and Main Results: CRSsNP and eosinophilic CRSwNP exhibited distinct miRNA expression profiles. MiR-125b was specifically up-regulated in eosinophilic CRSwNP. MiR-125b was mainly expressed by sinonasal and bronchial epithelial cells. EIF4E-binding protein 1 (4E-BP1) was identified as a direct target of miR-125b. MiR-125b mimic or inhibitor enhanced or decreased IFN-alpha/beta production elicited by dsRNA in vitro or in vivo, respectively. 4E-BP1 expression was decreased, whereas IFN regulatory factor-7 and IFN-beta expression was increased, in eosinophilic CRSwNP. IFN-beta mRNA levels positively correlated with IL-5 mRNA levels and eosinophil infiltration in sinonasal mucosa. IFN-beta stimulated B cell-activating factor of the tumor necrosis factor family production in airway epithelial cells. miR-125b could be induced by lipopolysaccharide, dsRNA, and IL-10. Conclusions: The up-regulated expression of miR-125b may enhance type I IFN expression through suppressing 4E-BP1 protein expression in airway epithelial cells, which potentially contributes to mucosal eosinophilia in eosinophilic CRSwNP.