Factor VIII-related antigen and von Willebrand factor are synthesised by and released from vascular endothelium. Acute increases in the plasma concentration of these proteins may reflect endothelial cell damage. We have thus measured the plasma concentration of factor VIII-related antigen and von Willebrand factor, together with procoagulant factor VIII, during the course of acute diabetic ketoacidosis in seven patients. In addition, evidence for qualitative changes in the factor VIII complex was sought. Plasma factor VIII-related antigen and von Willebrand factor were markedly increased (plasma factor VIII-related antigen at presentation, median 2.75 U/ml; von Willebrand factor 2.95 U/ml) and returned toward normal with clinical and biochemical resolution (plasma factor VIII-related antigen at clinical recovery, median 1.80 U/ml; von Willebrand factor 2.05 U/ml). Plasma procoagulant factor VIII followed a similar pattern, but levels were less elevated (plasma procoagulant factor VIII, at presentation, median 1.6 U/ml; at clinical recovery, 1.2 U/ml). Crossed immunoelectrophoresis and sodium dodecyl sulphate-acrylamide electrophoresis with autoradiographic identification of multimeric structure revealed no evidence of structurally abnormal factor VIII-related antigen in diabetic ketoacidosis. However, an extra peak on crossed immunoelectrophoresis ("pre-peak") was a feature in the acute phase ketoacidotic plasma in six subjects, and may represent aggregated factor VIII. Changes in plasma factor VIII are a feature of diabetic ketoacidosis and, whilst not specific to this condition, may be the result of endothelial cell damage.