AbstractIn the present investigation, the frequency and severity of periodontal ‘disease was assessed in a group of 71 patients with a mean duration of 16.5 years of insulin‐dependent diabetes mellitus (IDD). The diabetics, aged 17–63 years, were under treatment at the diabetic outpatient clinic of the III Department of Medicine, University Central Hospital of Helsinki and at two clinics of the Helsinki Health Centre. Based upon their long‐term medical records, 44 individuals were assessed to have a poorly controlled insulin‐dependent diabetes mellitus (PIDD). At baseline of the present study, the PIDD group had a mean blood glucose level of 11.8 mmol/1 and a mean glycosylated hemoglobin (HBA), level of 10.7%. 27 subjects were classified as having a controlled insulin‐dependent diabetes mellitus (CIDD). For each individual, site‐specific recordings were made for the plaque index, gingival index, pocket depth, loss of attachment, bleeding after probing, gingival recession and radiographic loss of alveolar bone. Under similar plaque conditions, adult subjects with a long‐term PIDD were found to have lost more approximal attachment and bone than subjects with a CIDD (P=0.046, P=0.019). These differences were not equally obvious when the subjects were classified according to the history of medical complications, such as retinopathies, neuropathies an