PURPOSE: Chronic rhinosinusitis (CRS) has a high incidence rate and different endotypes. Serum 25-hydroxyvitamin D (25(OH)D) deficiency is common in patients with eosinophilic and non-eosinophilic chronic rhinosinusitis with nasal polyps (ECRSwNP and nECRSwNP, respectively). This study explored the relationship between serum 25(OH)D levels and CRS risk and determined the value of combining serum 25(OH)D with peripheral blood markers in distinguishing between ECRSwNP and nECRSwNP. METHODS: This study enrolled 275 CRS patients and 298 healthy controls. The relationship between serum 25(OH)D levels and CRS risk was determined using logistic regression after propensity score matching (PSM). The efficiency of various peripheral blood markers in distinguishing between ECRSwNP and nECRSwNP was assessed using a decision-tree model. RESULTS: The final analysis included 189 CRS patients and 189 controls after 1:1 PSM. Serum 25(OH)D levels were significantly lower in CRS patients than in controls. Patients with mild CRS showed higher serum 25(OH)D levels than those with moderate or severe CRS, and ECRSwNP patients had lower serum 25(OH)D levels than nECRSwNP patients (all Ps <
0.05). Eosinophil percentages and IgE levels were independent risk factors for ECRSwNP, whereas serum 25(OH)D was an independent protective factor. 25(OH)D deficiency increased the ECRSwNP risk (OR = 3.074, P = 0.04). An eosinophil percentage ≥ 5% and a 25(OH)D level <
61.8 nmol/L could be used to predict ECRSwNP, with training and test set accuracies of 89.7% and 85.7%, respectively. CONCLUSION: Our findings suggest that serum 25(OH)D deficiency is an independent risk factor for CRS, especially ECRSwNP. Combining serum 25(OH)D levels with peripheral eosinophil percentages could be a promising biomarker for ECRSwNP.