BACKGROUND: Bronchial asthma and allergic rhinitis often concomitant to form a single clinical and pathogenetic syndrome based on the activation of T2 inflammation. Important biomarkers of this type of inflammation are eosinophils and total immunoglobulin E. Prolonged course of allergic rhinitis in patients with asthma can lead to the development of chronic rhinosinusitis with polypoid-hyperplastic changes in the sinonasal mucosa. The effect of polypoid-hyperplastic changes on the severity of clinical manifestations and the level of systemic biomarkers of T2 inflammation in patients with a combination of asthma and allergic rhinitis in the pediatric population cannot be considered established. AIM: Сomparаtive the clinical manifestations (assessed by TNSS and SNOT-22 scales) and the content of systemic biomarkers of T2 inflammation between groups with the presence and absence of polypoid-hyperplastic changes in children and adolescents with combined course of bronchial asthma and allergic rhinitis. METHODS: The single-stage, single-center observational study included patients with asthma and AR aged 6–17 years. All patients underwent clinical and laboratory examinations, rhino endoscopy and computed tomography of the nose and paranasal sinuses according to indications. RESULTS: Totally, 268 patients were examined: 203 (75.75 %) boys and 65 (24.25 %) girls. Polypoid-hyperplastic changes in the sinonasal mucosa were detected in 31.84 % of patients. Clinical symptoms (assessed using the TNSS and SNOT-22 scales) were more pronounced in children with polypoid-hyperplastic changes compared to those without polypoid-hyperplastic changes (p <0.001), but there was no significant difference between boys and girls in terms of these symptoms. At the same time, TNSS scores were statistically significantly higher for girls than for boys (p=0.045), and SNOT22 scores were significantly higher in both groups (p <0.05). Higher levels of eosinophils and total immunoglobulin E were also recorded by children with polypoid-hyperplastic changes (p <0.001 and p=0,001respectively), indicating a significant role for systemic T2 inflammation in this. CONCLUSION: The presence of polypoid-hyperplastic changes in the sinonasal mucosa by children and adolescents with bronchial asthma and allergic rhinitis correlates with increased clinical symptoms and increased levels of eosinophils in blood and total serum immunoglobulin E, especially in girls. The results obtained indicate the potential involvement of systemic T2-inflammation in the processes of pathological remodeling and the formation of structural disorders of polypoid-hyperplastic changes in the sinonasal mucosa. © 2025, Pharmarus Print Media. All rights reserved.