OBJECTIVE The aim: Of our research work was to study the level of proinflammatory interleukin-18 (IL-18) in the oral fluid of children with type I diabetes mellitus (DM), and to determine their periodontal status and the level of oral hygiene. PATIENTS AND METHODS Materials and methods: 82 children were examined, they were divided into groups by presence of gingivitis and diabetes mellitus. The level of interleukin-18 in oral fluid was determined by immunoassay. RESULTS Results: In patients with chronic catarrhal gingivitis and type I diabetes mellitus the level of interleukin-18 in oral fluid is the highest (70.91±7.48 pg / ml); the level of interleukin-18 in children with diabetes mellitus and healthy gums is high enough too, it is 14.87±1.11 pg / ml. Interleukin-18 is 3.41±0.25 pg / ml in healthy children with healthy gums. It is 5.74±0.27 pg / ml in somatically healthy children with chronic catarrhal gingivitis. CONCLUSION Conclusions: We indicated that an increase in the value of interleukin-18 in oral fluid is associated with the presence of diabetes mellitus in children. Moreover, this cytokine can be considered as a potential biomarker of gum inflammation in children with diabetes mellitus.