The importance of probiotics for the formation of pediatric immunity, their positive effect on the course of allergy, and their high safety formed the basis for this study of the impact of oral and intranasal co-administration of the active form of B. longum MC-42 on nasal breathing. Objective: to evaluate the impact of the active form of B. longum MC-42 on the nasal mucosa and nasal breathing in children with chronic adenoiditis and allergic respiratory diseases. Subjects and methods. An open-label randomized prospective comparative clinical study was conducted to examine 64 children aged 7—10 years with chronic adenoiditis in the presence of respiratory allergosis. The parents of the examined children were surveyed using a questionnaire. The trend in nasal symptoms was assessed by complaints and anterior rhinoscopic findings. Results. The children receiving the active form of the probiotic were observed to have significantly better positive changes in nasal symptoms, an overall decline in the species-specific range of the microflora, a reduction in the number of mixed microorganisms and in the degree of nasopharyngeal contamination, and an improvement in the body’s interferon status. Conclusion. Normalization of anti-infectious immunity via local and systemic mechanisms creates conditions for the lower bacterial contamination of the nasopharyngeal mucosa, and contributes to the substitution of pathogenic and rather highly virulent opportunistic strains for low-pathogenic saprophytes of the respiratory tract, which ultimately creates conditions for sustained remission and stable improvement of nasal breathing.