The most recent hypotheses on the pathogenesis of periodontal disease have underlined the great importance exerted by individual susceptibility to the disease. The ability of a high ratio between bleeding index and plaque index as a way of identifying subjects at risk has been investigated, the values of this ratio being compared in a group of diabetic patients and in a group of non-diabetics with periodontal lesions of similar extent. The results obtained confirmed those of other Authors, testifying to the good capability of the bleeding/plaque ratio to identify patients with a high risk of developing advanced forms of periodontal disease.