The prophylaxis of patients with hereditary angioedema to prevent attacks has gone through major revision as new agents for prophylaxis have come on the market. Earlier treatments, developed empirically, included the fibinolysis inhibitors epsilon aminocaproic acid and tranexamic acid and attenuated androgens such as danazol. With the development of these agents, many patients had relief of severe symptoms, and drugs in these classes have been the only treatments available in America. Their major disadvantage has been their side effects, which range from minor to severe. In Europe various products containing C1 inhibitor, the serum protein deficient in this disease, were prepared from pooled donor plasma. They were reported to be effective in ending attacks and in prophylaxis, but these products in general were not used in prophylaxis, in part because of the short half life of the plasma protein. One such product, C1 esterase inhibitor, has now been shown in a rigorous double-blind study to be effective in prevention of hereditary angioedema attacks and has been approved by the US Federal Drug Administration for prophylaxis of the disease. Its use has been attended by few side-effects, reflecting the fact that it is the purified naturally circulating product.