Type I von Willebrand disease (vWD) Vicenza is a rare variant with autosomal dominant transmission, characterized by the presence of supranormal von Willebrand factor (vWF) multimers in plasma, similar to those normally found in endothelial cells and megakaryocytes. The patients have very low levels of plasma vWF contrasting with a mild bleeding tendency. The pathophysiology of this subtype is still unknown. The presence of supranormal multimers in the patients' plasma could be due to a mutation in the vWF molecule which affects post-translational processing, or to a defect in the cells' processing machinery, independent of the vWF molecule. In order to determine if type I vWD Vicenza is linked to the vWF gene, we studied six polymorphic systems identified within the vWF gene in two apparently unrelated families with type I vWD Vicenza. The results of this study indicate a linkage between vWF gene and the type I vWD Vicenza trait. This strongly suggests that type I vWD Vicenza is due to a mutation in one of the vWF alleles, which results in an abnormal vWF molecule that is processed to a lesser extent than normal vWF.