The physical act of operating on 13 patients with meningiomas was studied for its effect on the fibrinolytic system. Fibrinolytic abnormalities, mainly due to an increase of plasma fibrinolytic activity, appeared in three patients prior to, during, or after surgery. These patients demonstrated hemorrhagic diathesis in the operative wounds which was associated with a consumptive coagulopathy, namely, an increase of fibrin/fibrinogen degradation product concentration and a decrease of fibrinogen concentration in plasma. Antiplasmin agents (gabexate and tranexamic acid) were effective in minimizing loss of blood during and after the operation. Abnormal hyperfibrinolysis seems to play a role in hemostatic difficulties in patients undergoing surgery for meningioma.