Hereditary angioedema is manifested by attacks of swelling of the extremities, face, trunk, airway, or abdominal viscera, occurring spontaneously or secondary to trauma. It is inherited as an autosomal dominant trait and is due to deficient activity of the inhibitor of the activated first component of complement. The clinical diagnosis can be confirmed by the findings of low levels of C4 or C1 esterase inhibitor activity, or both. Therapy may be divided into three phases: long-term prophylaxis of attacks, short-term prophylaxis of attacks, and treatment of acute attacks. Long-term prophylaxis may be achieved with antifibrinolytic agents and androgens. Short-term prophylaxis with these agents and plasma transfusions has been successful. Specific therapy for acute attacks is not available, but good supportive care, together with a knowledge of the course of the disease, can prevent asphyxiation from airway obstruction. Before the advent of therapy, mortality was reported as high as 30%.