To determine the relationship between cardiovascular complications of estrogen therapy and fibrinolysis, fibrinolysis parameters plasminogen, urokinase-type plasminogen activator (u-PA), tissue-type plasminogen activator (t-PA), and plasminogen activator inhibitor-1 (PAI-1), were assessed in 12 prostatic cancer patients before and 6 weeks after the onset of estrogen therapy. The levels of plasminogen, u-PA, and PAI-1 in the patients treated with the estrogen therapy were significantly higher than those in the patients before the therapy. The t-PA level in the patients during the therapy was significantly lower than that before the treatment. Cardiovascular complications were found in two patients (16.7%) during estrogen therapy. In the two patients, marked elevation of PAI-1 and decreased level of t-PA were observed during the therapy. These results indicate that cardiovascular complications of estrogen therapy in patients with prostatic cancer may be related to hypofibrinolysis resulting from changes of PAI-1 and t-PA.