Background Patients with angioedema experience unpredictable attacks of tissue swelling in which bradykinin is implicated. Several distinct mutations in Factor XII (FXII) are associated with hereditary angioedema (HAE) in the presence of normal C1 esterase inhibitor activity (FXII-HAE). The underlying disease mechanisms are unclear, which complicates diagnosis and treatment. Objective We sought to identify the natural trigger for FXII activation, which causes uncontrolled bradykinin production in patients with FXII-HAE. Methods We generated recombinant variants of FXII, representing health and disease, and studied their behavior in functional studies. We investigated bradykinin-forming pathways in blood plasma with newly developed nanobody-based analytic methods. Results We here report that FXII-HAE mutations collectively introduce new sites that are sensitive to enzymatic cleavage by plasmin. These FXII mutants rapidly activate after cleavage by plasmin, escape from inhibition through C1 esterase inhibitor, and elicit excessive bradykinin formation. Furthermore, our findings indicate that plasmin modulates disease activity in patients with FXII-HAE. Finally, we show that soluble lysine analogs attenuate this mechanism, explaining their therapeutic value in patients with HAE. Conclusion Our findings indicate a new pathway for bradykinin formation in patients with HAE, in which FXII is cleaved and activated by plasmin. This should lead to the identification of new markers for diagnosis and targets for treatment.