In this placebo-controlled trial, CSL830, a subcutaneous C1 inhibitor, significantly reduced the rate of hereditary angioedema attacks; local site reactions were the dominant side effect. Of the patients who received 60 IU per kilogram twice weekly, 40% had no attacks for 16 weeks. Hereditary angioedema is a disabling and potentially fatal condition characterized by recurrent episodes of swelling without urticaria or pruritus. The condition is caused by deficiency (type I) or dysfunction (type II) of the C1 inhibitor protein. 1 Patients have insufficient C1 inhibitor function to prevent bradykinin production by the contact system, leading to episodes of increased capillary hyperpermeability and swelling. These episodes manifest clinically as angioedema attacks. 2 , 3 Low levels of C1 inhibitor protein antigen or low functional levels of C1 inhibitor activity, as well as low levels of complement C4, are diagnostic for hereditary angioedema, and baseline C1 inhibitor function . . .