Objective <p>Conduct a cross-sectional study to investigate the periodontal conditions and oral health behaviours among adult subjects with type one diabetes mellitus and compare them with those of a group of age- and gender-matched without diabetes. Furthermore, we also intend to evaluate the potential indicators of the risk for the development of periodontal disease.</p> Methods <p>The evaluation was undertaken with patients with diabetes (<i>n</i> = 70) from a patients’ cohort of the the Hospitalar Center of Tâmega e Sousa and subjects without diabetes (<i>n</i> = 69).</p> Results <p>The prevalence of periodontal disease showed significant differences between groups. Gingivitis reached a prevalence of 37.1% in patients with type one diabetes mellitus and periodontitis 55.7%. These systemically compromised patients exhibited a higher prevalence of Plaque Index, Bleeding on Probing and Periodontal Probing Depth and a reduced number of teeth when compared to the control group. The stage II was the most prevalent in the diabetes population, followed by the stage IV and most of diabetes subjects presented grade C progression.</p> Conclusions <p>There is a higher prevalence of periodontal disease in type one diabetes mellitus individuals when compared to the controls. Age, Bleeding on Probing and number of cigarettes per day are associated with higher risk of periodontal disease in type one diabetes mellitus patients.</p> Clinical relevance <p>Our study provides evidence about the prevalence of periodontal disease among type 1 diabetes mellitus and creates awareness regarding the factors that potentially contribute to worsening periodontal tissues. Furthermore, informing diabetic patients about the importance of early diagnosis and prevention of periodontal disease and the importance of reducing/quitting smoking.</p>