A randomized double-blind placebo-controlled crossover trial of danazol was carried out in 19 cases of hemophilia A. Danazol was given for 3 months at a dose of 150 mg/day to patients under 15 years of age, and 300 mg/day to older patients. The basal factor VIIL·C level was 8.3 ± 5.6% (mean ± SD), and after 3 months of danazol treatment was 15.3 ± 11.0% (p = 0.02). Six patients (basal factor VIIL·C 2-22%) showed a 1.36- to 2.87-fold elevation of factor VIIL·C levels after danazol. 0/2,1/4 and 5/13 cases of severe, moderate and mild disease, respectively, responded. Decreases in the number of bleeding episodes and cryoprecipitate requirement were seen in the responders. No adverse reactions to danazol were encountered. Danazol appears to raise the factor VIIL·C levels in selected cases of hemophilia A.