Hereditary angioedema (HAE), characterized by recurrent episodes of angioedema involving the skin, or the mucosa of the upper respiratory or the gastrointestinal tracts, results from heterozygosity for deficiency of the serine proteinase inhibitor (serpin), C1 inhibitor (C1INH). The primary biological role of C1INH is to regulate activation of the complement system, the contact system, and the intrinsic coagulation system. During attacks of angioedema, together with decreasing levels of C1INH, the complement and contact systems are activated: C2 and C4 levels fall and high molecular weight kininogen is cleaved. Although previous data suggested that symptoms in HAE might be mediated via complement system activation, a combination of recent clinical data, in vitro studies, and analysis of C1INH-deficient mice all indicate that the major mediator of angioedema is bradykinin: (1) a vascular permeability enhancing factor can be generated in vitro in C1INH-depleted, C2-deficient plasma, but not from C1INH-depleted, contact system-deficient plasma; this factor was identified by sequence analysis as bradykinin; (2) bradykinin can be detected in the plasma of HAE patients during attacks of angioedema; (3) in several members of one family, expression of a C1INH variant that inhibits contact system proteases but has defective inhibition of C1r and C1s does not result in HAE; (4) C1INH-deficient (C1INH-/-) mice have a defect in vascular permeability that is suppressed by treatment with specific plasma kallikrein inhibitors and by bradykinin type 2 receptor (Bk2R) antagonists, and is eliminated in C1INH-/-, Bk2R-/- double-deficient mice.