Objective: We investigated the association between glycemic control of type 2 diabetes mellitus (type 2 DM) and severe periodontal disease in the US adult population ages 45 years and older. Methods: Data on 4343 persons ages 45-90 years from the National Health and Nutrition Examination Study III were analyzed using weighted multivariable logistic regression. Severe periodontal disease was defined as 2 + sites with 6 + mm loss of attachment and at least one site with probing pocket depth of 5 + mm. Individuals with fasting plasma glucose >126 mg/dL were classified as having diabetes; those with poorly controlled diabetes (PCDM) had glycosylated hemoglobin >9% and those with better-controlled diabetes (BCDM) had glycosylated hemoglobin less than or equal to9%. Additional variables evaluated in multivariable modeling included age, ethnicity, education, gender, smoking status, and other factors derived from the interview, medical and dental examination, and laboratory assays. Results: Individuals with PCDM had a significantly higher prevalence of severe periodontitis than those without diabetes (odds ratio = 2.90; 95% Cl: 1.40, 6.03), after controlling for age, education, smoking status, and calculus. For the BCDM subjects, there was a tendency for a higher prevalence of severe periodontitis (odds ratio 1.56; 95% Cl: 0.90, 2.68). Conclusion: These results provide population-based evidence to support an association between poorly controlled type 2 diabetes mellitus and severe periodontitis.