The effects of parenterally given polyestradiol phosphate (80 or 160 mg i.m. monthly) and bilateral subcapsular orchiectomy on blood coagulation and fibrinolytic parameters were compared in 11 patients with prostatic carcinoma. Estrogen therapy lowered antithrombin III, plasminogen and plasminogen activator inhibitor activities, whereas these parameters remained unchanged in orchiectomized patients. There were no significant changes in platelet count, fibrinogen, factor VII, protein C and alpha 2-antiplasmin in either group. Estrogen had unfavorable effects on hemostatic laboratory parameters in the direction of a hypercoagulable state. However, no thromboembolic complications were encountered.