Von Willebrand factor antigen (VW FAg) has been used as a marker for vasculitis and raised levels have been shown to correlate with active disease. The use of VW FAg levels in the diagnosis of vasculitic disorders has been proposed. However, certain cases of vasculitis have normal VW FAg levels and are difficult to diagnose; this is of importance because of the high mortality associated with these disorders. It has been suggested that venostatic stress can be used as a provocative test for stimulating VW FAg release, thereby improving the speed and sensitivity of diagnosis. We tested this hypothesis in a mixed group of 35 patients with vasculitis, systemic lupus erythematosus and rheumatoid arthritis and 16 controls. At baseline, although the levels were not outside the laboratory range, the disease groups had raised VW FAg compared with the simultaneously tested controls. Venostatic stress increased VW FAg activity in all disease groups, control levels also increased and differences between controls and disease groups diminished in significance. Therefore venostatic stress with VW FAg measurement does not produce a more sensitive test for vasculitis. Venostasis should be avoided when measuring VW FAg levels.