Abstract In the present two-year longitudinal investigation, the progression of periodontal disease was assessed after 1 year from the baseline examination in 38 dentate subjects and after 2 years in 22 dentate subjects with a mean duration of 18 years of insulin-dependent diabetes mellitus. The diabetics, aged 35 to 56 years at baseline, were under medical treatment at the outpatient clinic of the III Department of Medicine, University Central Hospital of Helsinki and at 2 diabetic clinics of the Helsinki Health Centre. Based upon their long-term medical records, 26 subjects were at baseline identified as having poorly controlled insulin-dependent diabetes (PIDD) with a mean blood glucose level of 12.5 mmol/1 and a mean glycosylated hemoglobin (HBA1) level of 10.1%. 12 subjects were classified as having controlled insulin-dependent diabetes (CIDD) with a mean blood glucose level of 6.7 mmol/1 and a mean HBA, level of 9.2% at baseline. For each individual, recordings were made at baseline and after 1 and 2 years from the baseline for the plaque index, gingival index, pocket depth, loss of attachment, bleeding after probing, gingival recession, and radiographic loss of alveolar bone. At baseline and 2 years after the baseline examination, the PIDD subjects had similar plaque conditions as the CIDD subjects. At baseline and after 1 and 2 years from baseline the PIDD subjects had more gingivitis and bleeding after probing (P<0.05, χtest) than the CIDD subjects. Both at baseline and at the 1-year and 2-year examinations, the long-term PIDD subjects had lost more tooth attachment and approximal bone than the corresponding CIDD subjects (P<0.01, χ-test). At all 3 examinations, the PIDD subjects also exhibited more gingival recession than the CIDD subjects (P<0.05, χ-test).