Twenty type II (non-insulin-dependent) poorly controlled diabetics had tests of coagulation and platelet function performed while receiving high-dose sulphonylurea therapy and at 1 and 3 months following their conversion to insulin. Although no overall change in glycaemic control (assessed by glycosylated haemoglobin) was noted, a reduction in thrombin generation was observed, as judged by a significant fall in fibrinopeptide A concentrations. No changes in factor VIII coagulant activity (VIII:C), factor VIII-related antigen or antithrombin III were found. Glycosylated haemoglobin concentrations showed significant correlations with antithrombin III and factor VIII:C, suggesting that improved glycaemic control might lead to an improvement of antithrombin III function and lower factor VIII:C concentrations. No changes in platelet function were detected. The introduction of an insulin regimen that improves glycaemic control might lead to a reversal of the 'hypercoagulable state' found in type II diabetes.