Clinical observations have indicated the frequent development of thrombotic complications during diabetic ketoacidosis (DKA). This study aimed to examine whether haemostatic changes that could lead to a thrombotic tendency occur during ketoacidosis. Plasma levels of in vivo haemostatic markers reflecting activation degrees of the coagulation system, fibrinolytic system, platelets and endothelium were assayed in 34 patients with DKA, both at diagnosis and 1 week after recovery. We found coagulation system and platelet activation and endothelial injury/activation in the patients at diagnosis of DKA. Although significant improvements were observed after recovery, only platelet activity was completely normalized. Fibrinolytic activity was also increased, both at diagnosis and after recovery, compared to the control group. However, although coagulation activity was prominently increased at diagnosis compared to the recovery period, there was no change in fibrinolytic activity in the same periods; on the contrary, the fibrinolytic capacity of the endothelium was diminished at diagnosis of DKA compared to the recovery period, suggesting the presence of relative hypofibrinolysis during DKA. Indications for a role of hyperglycaemia in the emergence of haemostatic disturbances during DKA were observed.