von Willebrand protein (vWF) is reduced by dithiothreitol (DTT) and, as a result, is not detected by enzyme-linked immunosorbent assay (ELISA). Plasma samples from normal subjects, children with haemolytic uraemic syndrome (HUS), and adults with vasculitis and vWF prepared from endothelium were treated with DTT before vWF assay. vWF in HUS and vasculitis resembled the endothelial form in being resistant to reduction. DTT modification of the ELISA assay may be useful as a marker of disease severity in conditions associated with endothelial cell damage.