Mounting data support a causal connection between high-normal fibrinogen levels and atherosclerotic cardiovascular disease. There is clearly a thrombogenic component to atherosclerosis and the onset of clinical manifestations. This offers the possibility to better identify high-risk candidates and also to protect them by reducing blood fibrinogen concentration or blocking its action. The relationship of antecedent fibrinogen to the subsequent development of cardiovascular disease is examined, based on 18 years of surveillance of a cohort of 1274 men and women aged 47 to 79 years who participated in the Framingham Study. The association with the development of peripheral arterial disease and cardiac failure is now examined in addition to previously studied relationships to coronary heart disease and stroke. In men and women, there is a significant age-adjusted relationship of fibrinogen level to coronary heart disease and to cardiovascular disease in general. In women, a significant relationship to cardiac failure and peripheral arterial disease, but not to stroke, was also found. These data on women are unique as they are not available elsewhere. Age-adjusted cardiovascular, all-cause, and coronary heart disease mortality were all related to fibrinogen in both sexes. In men, fibrinogen impact was the greatest for stroke and the least for peripheral arterial disease. For women, the impact on coronary heart disease was greatest. The absolute risk for an elevated fibrinogen level was greatest for coronary heart disease in both sexes. Average fibrinogen values are higher in women and in persons with other risk factors, including hypertension, cigarette smoking, diabetes, obesity, and elevated hematocrit. However, there is an independent contribution of fibrinogen to cardiovascular disease in general and coronary disease in particular, on adjustment for coexistent risk factors. Fibrinogen enhances the risk of cardiovascular disease in hypertensives, diabetics, and cigarette smokers. About half the cardiovascular risk of cigarette smoking appears due to the higher fibrinogen values. Now, five prospective studies document the excess incidence of cardiovascular events in persons with elevated fibrinogen levels within the "normal range." Each standard deviation increase in fibrinogen is associated with a 30% increment of coronary heart disease in men and a 40% increase in women. Fibrinogen should be added to the list of major cardiovascular risk factors. Trials of intervention to lower fibrinogen in high-risk coronary candidates are needed.