Data from the Centers for Disease Control and Prevention indicate that more than 20 million people (approximately 7% of the population) in the United States have diabetes mellitus. Physicians often fail to examine the mouths and teeth of their patients, even though the condition of the mouth and teeth have clinical relevance for the treatment of patients with diabetes mellitus. The authors examine the current state of knowledge regarding periodontal disease and the effect of periodontal disease on worsening of glycemic control. They review several studies investigating how the management of periodontal disease affects the ability of patients to control symptoms of diabetes mellitus. The authors conclude with several recommendations for the treatment of patients with periodontal disease to improve glycemic control.