A mother and her son are described who both suffer from hereditary angiooedema. Both were treated with Danazol, a synthetic derivative of ethisterone. The prophylactic clinical effect seemed obvious, and also the serum concentration of the complement component C4, which had regularly been lower before treatment, became normal. However, no change of the serum concentration of the C1 esterase inhibitor could be demonstrated; this is in contrast to observations of other authors.