The aim of this study was to define a population of diabetics exhibiting an increased risk of developing severe periodontitis by comparing the medical status of 2 groups of diabetics, 1 with no/minor periodontal disease and 1 with severe periodontal disease. The case-control study consisted of 2 parts, a baseline study and a follow-up study. 39 case-control pairs were selected. They were adult, long-duration, insulin-dependent diabetics matched according to sex, age and diabetes duration. One individual in each pair (the CASE) exhibited severe periodontal disease while the other (the CONTROL) exhibited gingivitis or only minor alveolar bone loss. The median age of the cases was 58 years (range 36 to 70 years) and of the controls 59 years (range 37 to 69 years). The median disease duration in cases and controls was 24 years and 25 years, respectively. The median follow-up time was 6 years. The medical variables analysed were weight, insulin dose, systolic and diastolic blood pressure, vibratory threshold, triglycerides, total-cholesterol, HDL-cholesterol, creatinine, HbA1, proteinuria, ECG, retinopathy, stroke, transient ischemic attacks (TIA), angina, myocardial infarct, heart failure, hypertension, intermittent claudication, foot ulcer, death, cause of death, and smoking habit. Biochemical analyses and clinical variables used as a routine in the monitoring of diabetics failed to differentiate between diabetics with severe and minor periodontal disease. In the follow-up study, significantly higher prevalences of proteinuria and cardiovascular complications such as stroke, TIA, angina, myocardial infarct and intermittent claudication were found in the case group. An association between renal disease, cardiovascular complications and severe periodontitis seems to exist. This indicates that a closer cooperation between the diabetologist and the dentist is necessary in monitoring the diabetic patient.