A computerized diabetes register was used retrospectively to define trends in HbA1 and blood glucose with increasing age, using a population of patients with insulin-dependent diabetes. A total of 1874 patients provided 11,776 results for analysis between 1982 and 1993. Mean HbA1 rose progressively between ages 10 and 16 years from 9.8% +/- 2.4 (SD) to 10.6% +/- 2.7 at 12, 11.7% +/- 3.1 at 14 and 12.2% +/- 3.2 at 16 years. Mean HbA1 then fell steadily until it reached a nadir of 10.2% +/- 2.8 at 27 years. There followed a gradual deterioration in control each decade: 10.2% +/- 2.9 at 30 years, 10.4% +/- 2.8 at 40, 10.9% +/- 2.6 at 50 and 10.8% +/- 2.7 at 60 years (r2 = 0.56). Glycaemic control was worse in females than in males: HbA1 area under curve between the ages of 10 and 70 years being 6.1% higher (p < 0.0001). This study confirms the clinical impression of worsening control during teenage years but provides evidence of previously unrecognized trends in later life. Nevertheless, a continuing improvement in glycaemic control between 1982 and 1993 was evident in all age groups.