The present invention relates to a method of diagnosing hereditary angioedema type III (HAE III) or a predisposition thereto in a subject being suspected of having developed or of having a predisposition to develop a hereditary angioedema type III or in a subject being suspected of being a carrier for hereditary angioedema type III, the method comprising determining in vitro from a biological sample of said subject the presence or absence of a disease-associated mutation in a nucleic acid molecule regulating the expression of or encoding coagulation factor XII; wherein the presence of such a mutation is indicative of a hereditary angioedema type III or a predisposition thereto. The present invention also relates to a method of diagnosing hereditary angioedema type III (HAE III) or a predisposition thereto in a subject being suspected of having developed or of having a predisposition to develop a hereditary angioedema type III or in a subject being suspected of being a carrier for hereditary angioedema type III, the method comprising assessing the presence, amount and/or activity of coagulation factor XII in said subject and including the steps of: (a) determining from a biological sample of said subject in vitro, the presence, amount and/or activity of: (i) a (poly)peptide encoded by the coagulation factor XII gene; (ii) a substrate of the (poly)peptide of (i); or (iii) a (poly)peptide processed by the substrate mentioned in (ii); (b) comparing said presence, amount and/or activity with that determined from a reference sample; and (c) diagnosing, based on the difference between the samples compared in step (b), the pathological condition of a hereditary angioedema type III or a predisposition thereto. The present invention also relates to a method of identifying a compound modulating coagulation factor XII activity which is suitable as a medicament or a lead compound for a medicament for the treatment and/or prevention of hereditary angioedema type III, the method comprising the steps of: (a) in vitro contacting a coagulation factor XII (poly)peptide or a functionally related (poly)peptide with the potential modulator; and (b) testing for modulation of coagulation factor XII activity, wherein modulation of coagulation factor XII activity is indicative of a compound's suitability as a medicament for the treatment and/or prevention of hereditary angioedema type III. Furthermore, the present invention relates to gene therapy methods and to a kit for diagnosing hereditary angioedema type III.