von Willebrand's disease (vWD) arises from abnormalities in von Willebrand factor (vWF), an adhesive glycoprotein uniquely involved in key aspects of both primary and secondary hemostasis. The current classification distinguishes disorders arising from partial (type 1) or complete (type 3) deficiencies and from qualitative defects (type 2). Type 2 vWD is further divided into four subtypes (A, B, N, and M), reflecting distinct classes of functional abnormalities. Mis-sense mutations account for most of type 2 vWD, whereas major disruptions in the vWF gene produce type 3 variants. The molecular basis of type 1 vWD is largely undefined. The laboratory diagnosis of vWD and its several variants is made on the basis of immunologic and functional studies of vWF, factor VIII levels, and specialized electrophoretic analysis (multimer gels). The mainstay of therapy for most patients with vWD in desmopressin, a pharmacologic agent that stimulates the release of endogenous pools of vWF. Cryoprecipitate and selected factor VIII concentrates are useful sources of exogenous vWF for the treatment of patients unresponsive to this desmopressin.