We gave danazol (600 mg/day orally for 14 days) to eight adults with mild or moderate hemophilia A, one with severe hemophilia A, and one with moderate hemophilia B. In the patient with severe hemophilia A, the levels of factor VIII two to four days after an infusion of factor VIII concentrate were higher than expected, suggesting a prolonged half- life. In one patient with mild hemophilia A, a questionable slight increase in factor VIII was noted at the end of the study. No change was seen in factor levels of other subjects. Therapy was terminated early, at eight days, in a patient who developed severe muscle cramps, and at ten days in a patient with a severe rash. Another patient developed hepatic dysfunction three days after completing the 14-day trial. In this trial, the side effects of danazol outweighed its meager and questionable benefits.