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Background. Upper respiratory tract infections (URTIs) are one of the most common diseases of childhood. It is known that some children tend to have recurrent bacterial complications of URTIs; the liability mentioned might be linked to different factors including local mucosal immune resistance malfunction. Infectious respiratory pathology is currently being considered comprehensively in the aspect of viral-bacterial-immune interaction, consequently the study of microbiological and immunological factor relations may be helpful in promoting the development of targeted therapeutic and preventive measures that may improve the efficiency of the management of such patients. Thereby, the study objectives were to investigate the local mucosal resistance state, features of microbial colonization of upper airways along with etiological structure complications in children susceptible to repeated bacterial complications of URTIs. Materials and methods. We enrolled 214 children aged 3 to 18 years with URTIs complicated with acute otitis media (AOM) or acute bacterial rhinosinusitis (ARS). Rate of bacterial complications of URTIs in 128 children was low (group I) and in 86 children it met the criteria of recurrent course (group II). We performed microbiological examination of nasopharyngeal secretions or middle ear exudates as well as local mucosal immunology study: lysozyme, lactoferrin, human cathelicidin hCap-18/LL-37, secretory IgA concentrations in oropharyngeal discharge measured twice during the disease, and after recovery. The controls were 36 children with uncomplicated URTIs, and the reference values were obtained in 30 healthy children. Results. In children with recurrent (versus episodic) bacterial complications of URTIs malfunction of airway local protective innate mucosal immune mechanisms were present: reduction of human cathelicidin level during the whole period of disease, reduction of lactoferrin at the early stages of bacterial process, and reduced concentrations of lysozyme at all phases of the study. It’s noteworthy that concentrations of hCAP/LL-37 and lactoferrin during the disease exceeded control and referent values in both groups; while lysozyme levels obtained from children of the II group were significantly lower compared to referent patients both initially and after recovery. The levels of antimicrobial peptides demonstrated the inverse correlation with URTI symptoms duration. In the II group the level of negative microbiological results for causative pathogen of AOM/ARS was about 35 % vs. about 20 % in group I (p = 0.008). Also lower rate of Streptococcus pneumoniae and nontypeable Haemophilus influenzae (pathogenic bacteria), and higher rate of Haemophilus parainfluenzae and Staphylococcus aureus (associated agents) isolates occurred in group II. Furthermore, associations of two non-causative bacterial agents were obtained only in group II. Non-causative bacteria colonization in general was associated with lower levels of mucosal antimicrobial peptides, but on the background of URTI it might activate tempora­rily lysozyme and lactoferrin response. Conclusions. Recurrent bacterial complications of URTIs are associated with local mucosal resistance insufficiency, and higher rate of St.aureus and H.parainfluenzae colonization. High rate of microbiological failures in determining of causative agent of AOM/ARS may be related to over diagnostics of bacterial complications which might be linked to misinterpretation of prolonged course of the disease due to mucosal immune dysfunction and non-causative bacteria colonization. Thus, di­sease duration shouldn’t be used as an independent criterion of bacterial process in this category of patients because it may lead to misdiagnose and antibiotic overuse.