Allergic vasculltls phenomena seem to be involved in Henoch-Schtjnlein purpura (HSP). Elevated plasma levels of von Willebrand factor (vW9 are a well recognized feature of vasculitis and have been taken as an indication of In vivo endothellal cell damage. Plasma factor VI1I:C and vWf levels and vWf multimeric pattern were studied in 8 patients with HSP, during active disease and twice during the remission (3 and 9 months later). Plasma vWf multimerlc composition was evaluated using low resolution gels whlch better resolve large vWf multimers. During active disease plasma factor VIII:C, vWf:Ag, and vWf:RCoF were normal in 5% of patients and Increased In three, but In each patient, platelets appeared to aggregate at doses of ristocetin lower than In normals. Furthermore, all patients demonstrated the presence of abnormally large vWf multlmers usually found only in platelets and endothelial cells. Three and 9 months later, during remission, in spite of the normalization of factor VII1:C and vWf levels, the abnormal multimers were still detectable, as well as hyper-responsiveness to ristocetin. These findings confirm that damage and/or perturbation of endothelial cells is associated with HSP. Moreover, the persistence of abnormality in the vWf multimeric pattern, when the disease is Inactive, suggests that the mechanisms Involved operate through the entire clinical courw.