The prevalence of von Willebrand's disease (VWD) in Western European countries and Israel was assessed. Possible patients were identified initially by questionnaire and those with plasma levels of von Willebrand factor antigen ( WFAg ; factor VIII related antigen) less than 1 u/dl (less than 1%) were confirmed by immunoradiometric assay performed at four reference centres. The prevalences of severe VWD in Scandinavian countries were similar to each other and about 10 times greater than those in other Western European countries. The prevalence in Israel was intermediate. The Registry will form the basis for future non- invasive epidemiological studies of atherosclerosis in these individuals.