Background There is clinical evidence for impaired lung function in chronic rhinosinusitis with nasal polyps (CRSwNP) patients, which may be due to a high incidence of asthma comorbidity. The lung function characteristics of non-asthmatic CRSwNP patients are not known. Small airway dysfunction (SAD) is involved in the pathogenesis of asthma. However, whether SAD is detected in non-asthmatic patients with CRSwNPs remains unclear. Objective This study analysed the lung function of non-asthmatic patients with CRSwNPs and evaluated its clinical relevance in CRSwNPs. Methods The clinical data for 191 consecutive CRSwNP patients (73 asthmatic and 118 non-asthmatic) and 30 control subjects were prospectively collected. The patients were followed up for at least 3 years (mean [standard deviation], 42.47 +/- 8.38 months). Serum and tissue total IgE levels were measured in 95 and 93 patients, respectively. Tissue eosinophil counts were documented in 63 patients. Results Non-asthmatic CRSwNP patients had decreased forced expiratory flow at 75% of the FVC (FEF75) and FEF(50)compared to the control subjects, and this difference was related to the severity of CRSwNP. The risk factors for impaired lung function in asthmatic and non-asthmatic patients were duration of asthma and smoking. A multivariate logistic analysis showed that decreased FEF(50)was associated with the recurrence of non-asthmatic CRSwNPs. The lung function of CRSwNP patients negatively correlated with the degree of type-2 inflammation, which was defined by the levels of Eos and IgE in polyp tissues and blood. The SAD of non-asthmatic CRSwNP patients was related to serum IgE levels. Conclusions and clinical relevance This study provides evidence that non-asthmatic CRSwNP patients may have SAD, which correlated with the severity and recurrence of CRSwNP. The decreased lung function of patients with CRSwNP was related to the degree of type-2 inflammation.