C1 inhibitor (C1INH), belonging to the superfamily of serine protease inhibitors, regulates not only complement system, but also the plasma kallikrein-kinin system, fibrinolytic system and coagulation system. The biologic activities of C1INH can be divided into the regulation of vascular permeability and anti-inflammatory functions. In recent years, hereditary angioedema (HAE), caused by an inherited deficiency of C1INH, has been focused. During HAE attacks, vascular permeability was markedly increased, which leads to angioedema. In sepsis, significant endothelial hyperpermeability is similarly observed systemically, but the role of C1INH has not been clarified in the pathogenesis. The serial change of C1INH in patients with sepsis is not clear. The objective of this study was to clarify the serial change in C1INH in patients with sepsis and evaluate the impact of C1INH on their clinical course.