Periodontal disease has been considered as a complication of diabetes mellitus. A recent epidemiological study revealed that obesity is an independent risk factor for periodontal disease. Obesity and type 2 diabetes mellitus are associated with many metabolic disorders including insulin resistance, dyslipidemia, hypertension and atherosclerosis. Chronic sub-clinical inflammation, although often for the most part in a healthy reference range, has recently been declared part of the insulin resistance syndrome, as such inflammatory responses appear to participate in the progression of metabolic disorders, including type 2 diabetes and atherosclerosis. We hypothesized that periodontal disease is one such sub-clinical inflammation. Here, we summarize current knowledge supporting this concept primarily based on data obtained from our own studies and propose a new concept that periodontal disease should be considered as part of the insulin resistance syndrome.