Plasma fibrinogen concentration is becoming widely accepted as a risk factor for cardio- and cerebrovascular disease. In order to evaluate its age-dependency, plasma fibrinogen and several other blood components were determined in 156 persons (23-96 years). In 116 participants, living on their own and rating themselves as healthy, fibrinogen increased by 26 mg/dL and decade. After exclusion of persons with pathological laboratory findings and risk factors, fibrinogen still rose by 25 mg/dL per decade, thereby reaching concentrations above 320 mg/dL in 81% of those older than 65 years. In prospective studies, these values were found to be associated with an augmented cardio- and cerebrovascular risk. In this investigation, the age-dependent rise of fibrinogen was more pronounced than that of blood glucose or cholesterol. Increasing fibrinogen may reflect a multitude of processes, such as (occult) acute phase reactions, risk factors, subclinical degeneration of vascular endothelium, or a more frequent activation of coagulation and fibrinolysis, with interleukin 6 (IL-6) as a possible mediator.