Hereditary angioedema (HAE) is a rare genetic disorder caused by mutations to C~1~-esterase-inhibitor (C~1~-INH) located on chromosome 11q and inherited as an autosomal dominant trait. HAE is characterized by low levels of C~1~-INH activity and low levels of C~4~. C~1~-INH functions to regulate the activation of the complement and intrinsic coagulation (contact system pathway) and is a major endogenous inhibitor of plasma kallikrein. The kallikrein-kinin system is a complex proteolytic cascade involved in the initiation of both inflammatory and coagulation pathways. One critical aspect of this pathway is the conversion of high molecular weight (HMW) kininogen to bradykinin by the protease plasma kallikrein. In HAE, normal regulation of plasma kallikrein activity and the classical complement cascade is therefore not present. During attacks, unregulated activity of plasma kallikrein results in excessive bradykinin generation. Bradykinin is a vasodilator which is thought by some to be responsible for the characteristic HAE symptoms of localized swelling, inflammation, and pain. Ecallantide is a potent (K~i~=25 pM), selective, reversible inhibitor of plasma kallikrein. Binds to plasma kallikrein and blocks its binding site, inhibiting the conversion of HMW kininogen to bradykinin and thereby treats symptoms of the disease during acute episodic attacks of HAE.