Heparin cofactor II (HCII) is a glycoprotein in human plasma which rapidly inactivates Thrombin in the presence of dermatan sulfate. Inhibition occurs by formation of a stable equimolar complex between HCII and Thrombin. Certain clinical conditions, such as hepatic failure, disseminated intravascular coagulation, thalasemina and sickle cell anemia, display reduced levels of HCII. However, during pregnancy, physiological levels of HCII expression are elevated. HCII may regulate coagulation and may participate in processes such as inflammation, atherosclerosis and wound repair. HCII is widely distributed among vertebrates and may have a common function in birds, amphibians and mammals.