Von Willebrand factor (vWf) is a multimeric and multivalent adhesive protein which is essential for platelet adhesion to subendothelium and for stabilization of factor VIII procoagulant activity in circulation. The quantitative measurement of vWf involves essentially two different approaches. The first is based on the interaction between vWf and Gp Ib of the platelet membrane in presence of ristocetin (ristocetin cofactor activity, RiCof) and depends not only on the amount of the factor but also on its ability to bring about this interaction, large multimers being more active. The second approach involves the immunological quantitation of vWf (vWf:Ag) by its interaction with specific polyclonal or monoclonal antibodies as measured by several methods, i.e., electroimmunoassay, immunoradiometric assay and immunoenzymatic assay. Although in the majority of type II von Willebrand disease (vWd) with dysfunctional vWf there is a discrepancy between RiCof and vWf:Ag, it should be emphasized that RiCof activity does not entirely reflect the 'true' activity of vWf since it does not explore all the functions of this factor; furthermore, the relationship between degree of multimerization and RiCof level is not always tenable, as for example in vWd 'Vicenza'. For the diagnosis of congenital and acquired vWd RiCof assay together with family investigation is the eligible test, with an estimated ability to detect at least 50% of the carriers of the abnormal gene, including mildly affected patients; vWf:Ag appears less sensitive and, on the basis of studies carried out in our laboratory, a relative sensitivity of 64% is proposed. Both assays require the definition of separate normal ranges for children and adults and for 0 and non-0 blood group subjects; a nonparametric approach in a large sample of normal subjects is advisable. With RiCof assay performed by an aggregometric method using formalin-fixed platelets an interassay variability of 6% and 8.5% respectively for high- and low-control plasma was found in our laboratory. With vWf:Ag assayed by an ELISA method a variability of 7% for low- and 6% for high-control plasma was found. Thus, both methods appear sufficiently precise for clinical use. The use of an internal pool calibrated against an international standard allows to perform comparable interlaboratory measurements. To further improve standardization of these assays, collaborative studies seem urgently required.