The aim of our study was to analyze inflammatory pathology of periodontal tissues in patients with diabetes mellitus, and the relationship of this pathology with other complications caused by diabetes mellitus. In our study, we evaluated 126 people aged 16-53 years (42 males and 84 females) with diabetes mellitus admitted to the Clinic of Endocrinology of the Hospital of Kaunas University of Medicine (HKUM). The condition of periodontal tissues was evaluated according to the World Health organization (WHO) CPITN index. Oral hygiene was evaluated using a simplified oral hygiene index (OHI-S) according to Green-Vermillion. Out of 126 subjects with diabetes mellitus, periodontitis was detected in 96 patients (36 males and 60 females) (CPITN index 2-5). Gingivitis was found in 27 subjects (CPITN index 1). Only 2.4% of the studied patients had healthy periodontal tissues. The study analyzed complications of other organs (neuropathy, and nephropathy and retinopathy) caused by diabetes mellitus. The obtained findings showed that microvascular complications were diagnosed more frequently in the presence of more severe inflammatory pathology of periodontal tissues. Retinopathy was diagnosed in patients with CPITN index 2.8+/-0.1, while patients with CPITN index 1.8+/-0.3 had no retinopathy. Neuropathy was more common among patients whose CPITN index was 2.9+/-0.1, while the condition was absent in cases where the CPITN index was 1.8+/-0.2. Comparable results were yielded by the studies of nephropathies in relation with changes in periodontium. Nephropathy was diagnosed in patients whose CPITN index was 3.0+/-0.1, and was not found in patients with CPITN index 2.1+/-0.2. The generalization of the obtained study data allows for stating that a more detailed analysis of factors causing complications of diabetes mellitus will also allow for a more profound understanding of the etiopathogenetic mechanisms that cause inflammatory pathology of periodontal tissues.