Summary. Diabetes mellitus is characterized by a high risk of atherothrombotic events. What is more, venous thrombosis has also been found to occur more frequently in this patient group. This prothrombotic condition in diabetes is underpinned by laboratory findings of elevated coagulation factors and impaired fibrinolysis. Hyperglycemia plays an important role in the development of these hemostatic abnormalities, as is illustrated by the association with glycemic control and the improvement upon treatment of hyperglycemia. Interestingly, stress induced hyperglycemia, which is often transient, has also been associated with poor outcome in thrombotic disease. Similar laboratory findings suggest a common effect of acute vs. chronic hyperglycemia on the coagulation system. Many mechanisms have been proposed to explain this prothrombotic shift in hyperglycemia, such as a direct effect on gene transcription of coagulation factors caused by hyperglycemia-induced oxidative stress, loss of the endothelial glycocalyx layer, which harbours coagulation factors, and direct glycation of coagulation factors, altering their activity. In addition, both chronic and acute hyperglycemia are often accompanied by hyperinsulinemia, which has been shown to have prothrombotic effects as well. In conclusion, the laboratory evidence of the effects of both chronic and acute hyperglycemia suggests a prothrombotic shift. Additionally, hyperglycemia is associated with poor clinical outcome of thrombotic events. Whether intensive treatment of hyperglycemia can prevent hypercoagulability and improve clinical outcome remains to be investigated.