Twenty patients with prostatic carcinoma were randomized to therapy with either oestrogens (n = 10) or orchidectomy (n = 10). Activators and inhibitors of coagulation were studied before treatment, 1.5 months and 6 months after the start of treatment. We found that the patients in the oestrogen group had already increased their factor VII level after 1.5 months (P less than 0.001) and this increased level persisted after 6 months. Factor X tended to increase after 1.5 months and this increase reached significance after 6 months (P less than 0.01). In the orchidectomy groups there was a significant increase in factor X at 6 months (P less than 0.01) and, in addition, antithrombin III (AT III) was increased at this time. Furthermore, there was a parallelism between the increase in factor VII and electrocardiographic evidence of increased coronary insufficiency (r = 0.60; P less than 0.025; n = 15). We found a significant increase of thromboxane as evidenced by the major urinary metabolite 2,3-dinorthromboxane B2 in the oestrogen group as compared to the orchidectomy group. In summary, patients with prostatic cancer during long-term oestrogen treatment were found to have increased levels of factor VII, factor VIII:C and fibrinogen. In addition these patients showed increased formation of thromboxane. The changes imply a hypercoaguable state and platelet activation. No such signs were found after orchidectomy. The findings in the oestrogen group might explain the continuously increased risk of cardiovascular complications during long-term oestrogen therapy.