A reasonable interpretation of the present evidence indicates that diabetes, when a complication of periodontitis, acts as a modifying and aggravating factor in the severity of periodontal infection. Diabetics with periodontitis who were young and poorly controlled, those who were long-duration diabetics, especially those over 30 years old, demonstrated more attachment loss, bone loss, and deeper probing pocket depths than their nondiabetic controls. It seems that the earlier the onset of diabetes and the longer the duration, especially without consistent control, the more susceptible the individual will be to periodontal disease. Consequently, once a diabetic contracts periodontal disease, it is usually more destructive. Although plaque scores of diabetics may be comparable to or even less than those of nondiabetics, diabetics often exhibit higher gingival index scores. The elevation of this particular clinical parameter is indicative of the microangiopathy associated with diabetes. Diabetic microangiopathy contributes to compromised delivery of nutrients to surrounding tissues and poor elimination of metabolic waste products. The complications associated with diabetes such as macroangiopathy, microangiopathy (i.e., retinopathy), ketoacidosis, and hyperglycemia result in impaired wound healing, immunosuppression, and susceptibility to bacterial infection. Individuals ages 30 to 40 suffering from diabetic retinopathy had significantly more gingival inflammation than controls or diabetics without complications. Collagen metabolism is defective in diabetics and is one component underlying delayed wound healing. Animal studies have been instrumental in elucidating the details of delayed wound healing. Hyperglycemia was associated with increased collagenase and protease activity in the gingiva of rats. Vascular wound healing in rats, particularly new re-endothelialization across vascular anastomoses, was significantly impaired. Diabetic abnormalities in immune response include impaired neutrophil chemotaxis, phagocytosis, and adhesion. Decreased neutrophilic chemotactic response seems to be attributable to protein factors in diabetic serum that competitively bind neutrophil receptors, thereby preventing complement-mediated phagocytosis. Because diabetics are not able to eliminate circulating immune complexes (CIC) effectively, serum CIC levels are elevated. There are microbiological differences in the characteristic flora of NIDDM patients and IDDM patients with periodontitis. These differences are not associated with diabetic impaired immune response. Ultimately, bacterial plaque is the primary etiology of periodontal diseases. Evidently, the host's response to bacterial plaque and ability to heal following surgery is altered by diabetic disease. Therefore, a thorough history regarding onset of diabetes, duration, and diabetic control would prove useful in the clinical management of diabetics presenting for treatment of periodontal disease.