An 8-year-old boy with hereditary angio-oedema was treated with danazol under close endocrinological supervision. The boy's C1 esterase inhibitor (C1inh) and C4 levels increased rapidly to near normal values under a daily dose of 400 mg and were maintained at about 50% of the normal by maintenance doses of 200 mg danazol every other day. Throughout the entire treatment period (11 months) the boy has been maintained free from attacks of angio-oedema. No hormonal imbalance was detected in the follow-up period. Our results indicate that danazol should be suitable for the treatment of HAE not only in adults but also in prepubescent children.