Oral contraceptives (OCs) raise the plasma levels of some clotting factors, especially factor VII. Until recently, however, there has been no evidence for a relationship between high levels of clotting factor and the onset of clinically manifest vascular disease. It has not been established, on a preliminary basis, that high levels of factors VII and VIII and of fibrinogen in men are associated with an increased risk of death from cardiovascular disease. There is a strong relationship between OC estrogen dose and the level of factor VII, a clotting factor with a number of other characteristics, suggesting that high levels would be thrombogenic. A strong relationship also exists between OC estrogen dose and the risk of thromboembolism. The thrombotic effects of OC are probably mediated, at least partly, through their effects on the coagulation system.