A 51-year-old man with recurrent episodes of angioedema was diagnosed as having autoimmuneacquired angioedema, based on adult onset, lack of apparent family history, decreased activity of Cl esterase inhibitor (Cl INH) and CH50, decreased levels of serum C4 and Clq and the presence of autoantibodies to Cl INH. The danazol treatment relieved the symptoms of angioedemaand increased the Cl INHactivity and concentration with the normalization of CH50, Clq and C4 levels. To our knowledge, this is the first case of autoimmune acquired angioedema in Japan. (Internal Medicine 41 : 398-402, 2002)