A great deal of evidence suggests that fibrinogen is implicated in many vascular disorders resulting from atherothrombosis. However, it has not yet been possible either to establish with certainty whether elevated plasma fibrinogen levels cause, or arise as a consequence of, vascular pathology; or to include fibrinogen in the battery of laboratory tests widely used to predict the risk of thrombosis. This article provides an overview of the progress that has taken place in recent years in our understanding of the pathogenetic role of high plasma fibrinogen levels; the identification of genetic markers of atherothrombotic disease; and the clarification of the complex epidemiology of plasma fibrinogen levels as it relates to genotype and environment. The recent improvement in methodology used to measure plasma fibrinogen levels (which should lead to the wider use of fibrinogen assays in clinical practice) is also reviewed.