Estrogen replacement therapy (ERT) appears to markedly reduce the risk of cardiovascular disease in postmenopausal women. There is evidence that estrogen effects on blood coagulation and fibrinolysis are important mediators of this beneficial effect. It is the acute phase reactants such as factor VII (F VII), von Willebrand factor (vWF) and fibrinogen (Fbg) as well as the main inhibitor of the fibrinolytic system, the plasminogen activator inhibitor (PAI 1), which have been shown to be associated with a particular predisposition or poor prognosis of cardiovascular disease. Additionally, the analysis of stabile reaction products of the coagulation cascade allows for an assessment of the loss of endothelial anticoagulant properties, i.e. endothelial injury. We compared the effects of oral versus transdermal ERT on these key parameters of the hemostatic system. 42 postmenopausal women were randomly assigned to receive either a novel transdermal system releasing 50 micrograms 17-beta-estradiol/24 hours or oral therapy with 0.6 mg conjugated estrogens combined with cyclic medrogestone 5 mg on day 11-21 for three treatment cycles. The study was performed according to the criteria of good clinical practise. We observed no adverse effects on the hemostatic system. Particularly, no increase of coagulatory reaction products, i.e. activity was found. Differences between groups were seen with regard to the extent of favourable effects: While the continuous transdermal ERT significantly reduced factor VII activity, oral ERT had no effect. However, oral ERT significantly reduced PAI 1 concentration by 40% suggesting an improved fibrinolytic capacity.(ABSTRACT TRUNCATED AT 250 WORDS)