Diabetes mellitus, caused by the malfunction of insulin-dependent glucose and lipid metabolism, presents with the classical triad of symptoms: polydypsia, polyuria, and polyphagia which are often accompanied by chronic fatigue and loss of weight. Complications of diabetes mellitus include retinopathy, nephropathy, neuropathy, and cardiovascular disease. Periodontal diseases are infections affecting the periodontium and resulting in the loss of tooth support. The association between diabetes mellitus and periodontitis has long been discussed with conflicting conclusions. Both of these diseases have a relatively high incidence in the general population (diabetes 1% to 6% and periodontitis 14%) as well as a number of common pathways in their pathogenesis (both diseases are polygenic disorders with some degree of immunoregulatory dysfunction). On the one hand, numerous reports indicate a higher incidence of periodontitis in diabetics compared to healthy controls, while other reports fail to show such a relationship. Clarification of this dilemma is occurring as the diagnostic criteria for periodontitis and diabetes mellitus improve, controlled studies with increased sample sizes are carried out, and the studies take into account major confounding variables that impact on the pathogenesis of both diseases. Current studies tend to support a higher incidence and severity of periodontitis in patients with diabetes mellitus. The overview looks at the bidirectional relationship between periodontitis and diabetes. An analysis of the National Health and Nutrition Examination Survey (NHANES) III data set confirms the previously reported significantly higher prevalence of periodontitis in diabetics than in non-diabetics (17.3% versus 9%). The analysis of the data also shows that the prevalence of diabetes in patients with periodontitis is double that seen in the non-periodontitis patients (12.5% versus 6.3%) and that this difference is also statistically significant. The pathogenesis of the 2 diseases is reviewed with an emphasis on common genetic and immune mechanisms. On the basis of the overview, 2 hypotheses for testing the relationship between periodontitis and diabetes are discussed. The first proposes a direct causal or modifying relationship in which the hyperglycemia and hyperlipidemia of diabetes result in metabolic alterations that may then exacerbate bacteria-induced inflammatory periodontitis. The second hypothesis proposes that a fortuitous combination of genes (gene sets) could result in a host who, under the influence of a variety of environmental stressors, could develop either periodontitis or diabetes or both.