Treatment with 17 alpha-methyltestosterone and with some synthetic androgens prevents attacks of hereditary angioedema (HAE). However, the potential hepatotoxicity of 17 alpha-alkylated androgens raises the problem of long-term prophylactic use of these agents. Therefore we compared the efficacy in preventing HAE attacks of 17 alpha-alkylated steroids (danazol and stanozolol) with non-17 alpha-alkylated derivatives (quinbolone, nandrolone decanoate and mesterolone). As the latter group proved ineffective, it seems that a drug's efficacy in preventing HAE attacks is connected to its 17 alpha-alkylation. Moreover, our long-term observations with the minimum effective dose of danazol seem to indicate the absence of important collateral effects.