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von Willebrand factor (vWF) is a large glycoprotein secreted predominantly by endothelial cells in both the systemic and pulmonary circulations and has a central role in the formation of the platelet plug. It has been put forward as a possible marker of endothelial cell injury, but is not ideal in that it is not specific for either the pulmonary or systemic circulation and may be released as part of the acute phase response from otherwise healthy endothelial cells. We undertook two studies (i) to assess within-subject to assess within-subject variation in plasma von Willebrand factor antigen (vWF:Ag) levels over time and to assess between-subject variation in a healthy patient population, and (ii) as part of a descriptive study of acute bronchitis, to assess whether plasma vWF:Ag levels altered in such a common and minor insult. A random sample of patients aged 45-74 years were taken from a local general practice. vWF:Ag levels were measured on three occasions, and spirometry was performed. The descriptive study was undertaken on patients in the general practice diagnosed with acute bronchitis without pre-existing pulmonary disease. Plasma vWF:Ag was measured on presentation and 14 and 42 days later. In 219 randomly selected patients the mean plasma vWF:Ag was similar at all three visits, the within-subject standard deviation being 0.09 U ml(-1) and 1.12 U ml(-1) respectively). There was no correlation between plasma vWF:Ag and C-reactive protein on presentation. We conclude that there is relatively little variation in an individual's plasma vWF:Ag level but that levels increase significantly with age. The observed elevation occurring with acute bronchitis is a true phenomenon; the absence of an associated acute phase response suggests that endothelial cell injury is the mechanism for the rise. These observations are important in the context of vWF as a marker of endothelial cell damage, as a common and supposedly minor insult such as acute bronchitis may markedly raise plasma levels.