Purpose: The present study is to investigate the effects of type I diabetes mellitus on dentition and oral health for children and adolescents. Materials and Methods: The investigation was carried out on 100 subjects. The first group consisted of 50 subjects with type I diabetes mellitus (21 females, 29 males), age 9 +/- 0.14 years; In the second group, there were 50 healthy subjects who did not suffer from any systemic disease (25 females, 25 males), age 9 +/- 0.11 years. The subjects were evaluated and divided into two groups of 5 - 9 years old, and 10 - 14 years old. The dentition of all participants was examined. Besides, the DFS/dfs index, oral hygiene conditions were evaluated, as well as the plaque index (PI), gingival index (GI) and calculus index (CI). The data obtained from each group were compared statistically. Results: When compared to the non-diabetic group, we observed that dental development was accelerated until the age of 10 in the diabetic group, and there was a delay after the age of 10. The edentulous interval was longer in the group with type I diabetes mellitus. This was accompanied by a high ratio of gingival inflammation. Gingival inflammation was 69.7% in the group of 5 - 9 year-old, and 83.7% in the group of 10 14 year-old with type I diabetes mellitus. Though there was a greater loss of teeth in the group with type 1 diabetes mellitus, there were more caries in the control group. The PI, GI and Cl values showed an increase with aging in favor of the group with type 1 diabetes mellitus. There was statistically significant difference in Pl, GI and CI between the control and type I diabetes mellitus groups for 10 - 14 year-old patients (p < 0.001). Conclusion: The findings we obtained showed that type I diabetes mellitus plays an important part in the dentition and oral health of children and adolescents.