The aim of the study was to evaluate the level of control, as reflected by HbA1c, in patients with diabetes attending one general practice over a 10-year period. The study was based in one general practice in South Tyneside, UK and consisted of an analysis of HbA1c values of all patients with diabetes attending the practice between 1983 and 1992. HbA1c levels were analysed and are presented as multiples of the standard deviation above the mean. In the practice 256 patients with non-insulin-dependent diabetes mellitus (NIDDM) and 76 with insulin-dependent diabetes mellitus (IDDM), attended for a total of 1596 doctor/patient contacts in the diabetic clinic over 10 years. The prevalence of diabetes was 1.9%. Over the course of the clinic, in any one year, 25% of patients with NIDDM and 55% with IDDM had levels of HbA1c above those thought to be associated with increased risk of microvascular complications. Significant reduction in glycated haemoglobin (HbA1c) occurred in the first year after diagnosis (p < 0.01) and after changing treatment from diet alone to diet and oral hypoglycaemic agents (p < 0.001). We conclude that a large proportion of patients within this population had levels of glycaemic control that put them 'at increased risk'.