Immune response in chronic rhinosinusitis with nasal polyps (CRSwNP) is mainly via type 2 T-helper (Th2) cells while Th1 cells characterise chronic rhinosinusitis without nasal polyps (CRSsNP) immune response. CRSwNP is heterogeneous on a cytological level causing a varied response to topical steroid therapy. One of the main subtypes of CRSwNP is Th2-biased eosinophillic inflammation. Interleukin 25 (IL-25) was shown to increase Th2 and eosinophillic activity. Tissue samples from nasal polyps of 92 patients with CRSwNP and from uncinate process of 20 CRSsNP patients and 16 controls were collected. IL-25 expression in the samples was quantified. Patients with CRSwNP were divided into two groups based on IL-25 expression and were compared. Nasal polyps cells were also cultured and effect of IL-25 on cytokine production in vitro was assessed. Findings showed increased levels of IL-25 in polypoid tissue when compared with controls. These results were supported with in vitro findings of Th2 activation when IL-25 was added to cells taken from nasal polyps. Patients with polyps containing higher levels of IL-25 scored more on their CT scans and on endoscopic (Lund-Kennedy) findings. The authors concluded that IL-25 could be targeted in future CRSwNP treatments due to its potential role in Th2 activation. This study adds to our understanding of CRS and paves the way for a new therapeutic possibility.