In the present study the influence of diabetes mellitus type 1 on periodontal tissues and teeth was evaluated in 44 patients. Ten randomly selected age and gender matched patients without diabetes served as a control group. Periodontal tissue was studied by Plaque Index, Gingival Index, Sulcus Bleeding Index, probing depth, epithelial attachment loss and alveolar bone loss. Teeth were evaluated by DMF counts. The presence of three periodotal pathogens, i.c. Actinobacillus actinmycetemcomitans, Prevotella intermedia and Porphyromonas gingivalis in subgingival plaque samples from two the deepest periodontal pockets in each patient was semi-quantitatively evaluated by the use of DNA probes. Results demonstrated that patients with diabetes have higher values of all examined parameters than patients without diabetes. The difference in sulcus bleeding index and loss of epithelial attachment between both groups was statistically significant. In the group of poorly controlled insulin-dependent diabetes mellitus patients all parameters were found to be higher than in the group of controlled insulin-dependent diabetes mellitus, though the differences were not significant. Degree of inflammation and destruction of periodontal tissue was statisticaly not significant higher in patients with microalbuminuria and retinopathia. Microbial analysis of subgingival plaque demonstrated increased levels of Prevotella intermedia and Porphyromonas gingivalis in diabetes mellitus patients in the samples taken from the sites with deep periodontal pocets. We are concluding that diabetes mellitus patients represent a high risk group for development of inflammation and periodontal disease.