Patients with diabetes mellitus have higher levels of coagulation factor VIII than the non-diabetic population. This may be a result of poor metabolic control and could contribute to the development of microvascular complications. During ketoacidosis there are acute changes in plasma concentrations of coagulation factors, some of which may be mediated by the rise in vasopressin that occurs. We have investigated the effects of hyperglycaemia without ketosis on some aspects of haemostasis by manipulating blood glucose concentrations using a Biostator. After a 1h run-in period with the blood glucose at 5 mmol/l, the blood glucose was maintained at 5, 15 and 25 mmol/l and maintained for one hour at each level in six male patients with insulin-dependent diabetes. Insulin was infused at 0.25 mu/kg/min. Venous blood samples were taken at the beginning and end of each hour after the run-in period for assays of factor VIII coagulant activity (FVIII:C), von Willebrand factor antigen (vWF:Ag), ristocetin co-factor (FVIIIR:Co), activated partial thromboplastin time (APTT) and vasopressin (aVP). There was a slight, though statistically insignificant fall in median factor VIII:C concentration at each incremental level of increase in blood glucose. Values (at the beginning and end of each hour) were: 1.0 and 1.1 iu/ml at 5 mmol/l; 0.95 and 0.79 iu/ml at 15 mmol/l; and 0.74 and 0.84 iu/ml at 25 mmol. vWF:Ag and FVIIIR:Co were unchanged. Plasma aVP fell slightly from 1.1 to 0.5 pg/ml. The results indicate that high levels of FVIII seen in diabetes are not due to short-term increases in blood glucose and that acute hyperglycaemia does not promote pro-coagulant changes in blood.