The biological syndrome in von Willebrand's disease is presently defined by four parameters: bleeding time, factor VIII coagulant activity (VIII:C), Willebrand factor activity (VIIR:WF) and factor VIII related antigen (VIIR:AG). The study of 48 patients from 25 families confirms the heterogeneity of the von Willebrand's syndrome including both quantitative as well as qualitative defects. Classical von Willebrand's disease results from the quantitative decrease of a plasma protein named "Willebrand factor" which is the primary gene-product of an altosomal chromosome. Both severe and moderate forms can be observed. In very severe forms, the apparent lack of the VIIIR:AG as measured by an immunoradiometric assay suggest the possibility that some patients are homozygous. "Variants" of von Willebrand's disease have been recently described in some patients, based on the discrepancy between the observed subnormal levels of VIIIR:AG and low levels of VIIIR:WF. The qualitative abnormality of the factor VIII/Willebrand protein can be assessed by different methods such as double cross immuno-electrophoresis, or immunoradiometric assay.