C1-inhibitor is the major control protein of the classical pathway of complement. In man, its hereditary deficiency results in the clinical condition of hereditary angio-oedema (HAE). The disease is characterised by recurrent episodes of acute, circumscribed, non-inflammatory oedema involving the subcutaneous tissues and mucous membranes. Acquired C1-inhibitor deficiency can also occur without any previous family history, at a later stage in life and is usually associated with a lymphoproliferative disorder or paraproteinaemia. C1-inhibitor deficiency is treated by the administration of anabolic steroids, such as danazol or stanozolol, which are known to increase hepatic synthesis of C1-inhibitor, resulting in an increase in plasma C1-inhibitor.