A genetic form of angioedema. Persons with it are born lacking the enzyme C1 esterase inhibitor, a protein that nor- mally inhibits the activation of a cascade of proteins. Without this inhibitor protein, angioedema occurs, resulting in recurrent attacks of swollen tissues, pain in the abdomen, and swelling of the voice box (larynx), which can compromise breathing. The diagnosis of hereditary angioedema is confirmed by finding subnormal blood levels of C1 esterase inhibitor. Treatment and prevention options include antihistamines and male steroids (androgens). Also known as hereditary angioneurotic edema.