Beside the well-known risk factors for atherogenesis fibrinogen is an independent risk factor. High plasma fibrinogen levels predispose to hypercoagulability and thrombotic processes. Furthermore, rheological mechanisms, interfering with cell-to-cell contacts, and fluid-dynamic factors, facilitate local endothelial lesions resulting in atherosclerosis. Plasma fibrinogen, as the major contributing factor to plasma viscosity and red-blood-cell aggregation can limit oxygen supply and blood flow in microcirculation even in the absence of apparent coronary artery disease. Pharmacological interventions, in order to decrease elevated fibrinogen levels, do improve myocardial ischemia in patients with severe coronary artery disease.