Elevated fibrinogen concentrations are recognized as playing an important role in the pathogenesis of atherosclerosis. In the framework of a risk factor survey in 342 middle-aged working men, screened twice over a period of five months, plasma fibrinogen levels were found to be fairly unstable as large discrepancies between both measurements were observed. Due to a substantial proportion of within-person variability, the reliability coefficient was only R = 0.56. Repeatability was highest in higher educated and physically more active men. Our data suggest that the impact of elevated fibrinogen levels on the development of ischemic heart disease and stroke, is likely to be under-estimated.