Summary. Eight insulin-treated diabetic patients in good glycaemic control were studied as out-patients with frequent determinations of stable glycosylated haemoglobin (HbAlc) before, during and after 1 week of induced poor glycaemic control. Stable HbAlo was determined by cation exchange chromatography after elimination of the labile fraction by incubation in saline (0.15 tool/l). The increase in mean blood glucose was significant on the first day of reduced insulin therapy and greatest after 1 Week (6.9___ 3.9 mmol/1 above basal values). Stable HbAac increased significantly on day 7 of the reduced insulin treatment. The increase represented, on average, 0.009% of total haemoglobin per mmol/1 increase in mean blood glucose per 24 h during the period of induced hyperglycaemia. After restoring insulin therapy, a significant decrease in blood glucose was achieved on day 1 and after 2 days, the blood glucose level was similar to before the study. There was no significant decrease in stable HbAlc within the first 2 weeks of improved glycaemia.