The coagulation factors and components of the fibrinolytic system were examined in 31 women with a previous history of phlebographically-verified thrombosis during the use of oral contraceptives of the combined type. Special attention was given to the histochemically-determined fibrinolytic activator content of the wall of biopsy specimens of superficial veins. None of the patients was taking contraceptives at the time of the investigation. Pathological changes, particularly in the fibrinolytic defence system, were found in most of the patients. They may be regarded as predisposed to thrombosis, and one might wonder whether these patients would sooner or later have had their thrombosis even if they had not used contraceptives. The concentration of antithrombin III was normal, indicating that this test is of no value for detecting patients predisposed to thrombosis, who should preferably not take oral contraceptives.