Twenty-one patients, 14 with haemophilia A and seven with haemophilia B, completed a double-blind crossover study to evaluate the effects of danazol on factor VIII and factor IX levels. Clotting and immunoradiometric assays were used to measure factor levels at baseline, 2 weeks and 8 weeks on both danazol and placebo. Fibrinogen, plasminogen and activated partial thromboplastin time were measured on all patients during placebo and danazol treatment. Although plasminogen levels rose significantly (P less than 0.01) and fibrinogen decreased (P less than 0.01), factor VIII and IX levels did not change. While on danazol, three patients had increased bleeding and shortened euglobulin lysis times compared to their baseline levels. We conclude that danazol does not raise factor VIII or IX levels and increases bleeding in some patients.