Aim – to study the species composition and population level of the microbiota of the content of the maxillary sinuses and the role of associations of microorganisms in the development and course of chronic purulent maxillary sinusitis (CPMS) in patients with type 1 diabetes.Material and methods. A microbiological examination of 97 samples of the contents of the cavity of the maxillary sinuses was carried out. The main group consisted of 50 patients with CPMS with type 1 diabetes at the age of 20-67 years (10 patients at the age of 20-31 years, 14 patients - 31-45 years old and 26 - at the age of 45-67 years). Among the patients of the main group, endocrinologists established the course of type 1 diabetes mellitus of moderate severity in 39 patients, and a severe course in 11 patients. The control group consisted of 47 CPMS patients of the same age without type 1 diabetes mellitus (DM).Results. Bacteriological and mycological methods in the content of maxillary sinuses of patients with maxillary sinusitis associated with type 1 diabetes mellitus (DM) 175 strains of different kinds of microorganisms were isolated and identified, belonging to 24 various taxonomic groups which form different by their qualitative content microbial associations in the biotope which consist of three different kinds in 58% of patients, consist of four kinds – in 34,0% and five various taxa – in 8,0% of patients. Chronic purulent maxillary sinusitis in patients with type 1 DM disrupts microbial associations. In patients with CPMS with type 1 DM in comparison with the control group, the number of associations consist of three kinds increases by 2,7 times, but the number of associations composed of 4 types of microorganisms decreases by 11.76%. The number of associations consisting of 5 types in patients is reduced by 3.5 times. The above may indicate the influence of not only the etiological agent but also a certain association of microorganisms on the severity of the course of CPMS associated with type 1 diabetes. All leading pathogens persist in the biotope in association. Associates, depending on their role in normobiocenosis, can inhibit the pathogenetic activity of the leading pathogen or, conversely, activate its pathogenetic role, which must be taken into account when choosing a therapeutic tactics.Conclusions. Chronic purulent maxillary sinusitis in patients with type 1 diabetes disrupts microbial associations. In the content of the cavity of the maxillary sinuses of patients with chronic purulent maxillary sinusitis, combined with type 1 diabetes mellitus, 175 strains of various types of microorganisms belonging to 24 different taxonomic groups were isolated and identified, which in the biotope form microbial associations of different qualitative composition, consisting of 3 different species in 58% of patients, out of 4 species in 34.0% and from five different taxa - in 8.0% of patients. Among the most numerous associations, consisting of 3 types of pathogenic and opportunistic autochthonous facultative microorganisms, associations of the following representatives are more common: M. catarrhalis, S. aureus and Bacteroides spp.; Prevotella spp., S. viridans and S. salivarius; M. catarrhalis, Prevotella spp. and S. epidermitidis; H. influenzae, Prevotella spp. and S. epidermitidis. Associations consisting of 4 species were found in 34% of patients and consist of S. pneumoniae, M. catarrhalis, S. pyogenes, Fusobacterium spp; S. pneumoniae, E. coli, S. aureus and Candida spp.; S. pneumoniae, E. coli Hly +, S. viridans and Candida spp. In patients with chronic purulent maxillary sinusitis, combined with type 1 diabetes mellitus with a severe course, there were associations consisting of S. pneumoniae, M. catarrhalis, Candida spp. and S. epidermitidis; S. pneumoniae, M. catarrhalis, S. pyogenes, S. epidermitidis; Bacteroides spp., H. influenzae, S. pyogenes, Enterobacter freundii spp.; Bacteroides spp., H. influenzae, S. pyogenes, Candida spp. In patients with chronic purulent maxillary sinusitis, against the background of a severe course of type 1 diabetes mellitus, associations of microorganisms, consisting of 5 types, were found. Their composition was different, but in all the pathogen S. pneumoniae was isolated and identified at a high population level, opportunistic obligate anaerobic bacteria of the genus Bacteroides and Prevotella, Fusobacterium, streptococci, and Staphylococcus aureus. All leading pathogens persist in the biotope in association, which must be taken into account when choosing therapeutic tactics.