Daily therapy and alternate-day therapy with the attenuated androgen oxymetholone were compared in patients with hereditary angioedema (HAE). Fifteen of 16 patients who experienced at least monthly attacks of HAE without treatment were asymptomatic on administration of 5 mg oxymetholene daily. When 13 of the patients who had been maintained asymptomatically on 5 mg oxymetholone daily were advanced to a treatment schedule of 5 mg every other day, seven attacks occurred during a cummulative 50 mo of therapy. The adverse effects that occurred with daily oxymetholone therapy largely subsided when the patients received alternate-day therapy, while a significant mean rise in C4 protein and function occurred only on daily therapy. Statistically significant mean increases in serum levels of C1INH occurred with daily therapy and were maintained with alternate-day therapy. Clinical benefit can be obtained with a treatment program that does not produce a statistically significant rise in C4 protein or function and does not raise C1INH to the lower limit of normal. The finding that alternate-day therapy diminished the side effects of the drug while affording a substantial reduction in the incidence and severity of attacks indicates the feasibility of this therapeutic approach.