Ischaemic heart disease and stroke are the major causes of death in the Western world. Established risk factors such as smoking, hypertension and hypercholesterolaemia explain only some of these events. Most myocardial infarctions and cardiac deaths are precipitated by acute occluding coronary thrombi, and it has been known for some time that thrombosis participates in atherogenesis. For these reasons, haemostatic variables have been included in studies of cardiovascular risk. The plasma fibrinogen level is associated with both the severity and the extent of coronary, cerebral and peripheral atherosclerosis. In prospective studies, fibrinogen was found to be an independent predictor of myocardial infarction in both sexes and of stroke in men. The plasma fibrinogen level thus provides information on risk over and above that supplied by established risk factors. Fibrinogen may play a part in atherothrombosis via several mechanisms: (1) by promoting atherosclerosis, (2) as an essential component of platelet aggregation, (3) because the amount of fibrin deposited and the size of the clot are directly related to the plasma fibrinogen level and (4) because fibrinogen increases plasma viscosity. Nevertheless, it is not yet possible to determine whether high fibrinogen levels are a cause or a consequence of cardiovascular disease because no drugs that selectively lower plasma fibrinogen levels are available. In addition, further standardization of measurements is needed before routinely including plasma fibrinogen in cardiovascular risk scores.