Abstract An analysis of haemostatic variables was done in 31 prostate cancer patients treated with oestrogens ( 13 pts), estramustine phosphate (7 pts) or orchidectomy (11 pts) before, at about 7 weeks and 6 months of treatment. Six patients treated with either of the drugs developed venous thromboembolism or ischemic vascular disease. Already before treatment there were changes indicating some activation of blood coagulation, fibrinolysis and kallikrein systems. The drug treated group showed significant changes in several variables: i.e. increase in factor VII, plasminogen and prekallikrein but also a decrease in antithrombin and in inhibitors to the fibrinolytic and kallikrein system. Significant difference between the drug treated groups was found in circulating platelet aggregates and in kallikrein inhibiting activity. Tissue plasminogen activator capacity was significantly lower in the drug treated patients with complications than in those without. The study also showed that in addition to the assay of the tissue plasminogen activator capacity during the first weeks of therapy it might be helpful in predicting cardiovascular complications to investigate platelet aggregates, prothrombin complex, factor X, von Willebrand factor antigen, fibrinogen, antithrombin, fibrinopeptide A, and the inhibitors of fibrinolysis as well as C 1-esterase inhibitor.