The authors discuss diagnostic difficulties in 12 cases of hereditary angioneurotic edema due to C1-esterase inhibitor (C1-INH deficiency). Emphasis is on the treatment of the acute attacks with intravenous infusions of C1-inhibitor concentrate (Boehring, West Germany). This proved to be a very efficient and safe therapy, leading to a prompt disappearance of all clinical symptoms. Throughout 12 months following the infusions, indices of the liver function remained within the normal range, and anti-Hbs and anti-HIV tests were negative.