Three families with von Willebrand's disease (vWd) type I were investigated. A reliable identification of healthy and diseased individuals was achieved by number of bleeding symptoms, assays of bleeding time, FVIII:C (one stage and two stage), VIIIR:Ag (EIA) and ristocetin cofactor. The diagnoses-vWd or non-vWd were confirmed by laboratory indices based on predictive values of positive and negative tests, also including VIIIR:Ag (IRMA and RIA). The last mentioned two variables did not contribute to significantly better identification of vWd versus health. The best single test variable for this purpose was ristocetin cofactor. One vWd family had significantly higher levels of ristocetin cofactor and shorter bleeding time than the other two vWd families and is probably the typical example of a family transmitting classical severe vWd.