A family with concurrent haemophilia A and type I von Willebrand's disease (vWd) is described. The propositus was affected by both disorders. The propositus' mother was an obligate carrier of haemophilia A being the daughter of a haemophilic. The father and sister were affected by vWd. The sister was also a possible carrier of haemophilia A. This is the first report of both disorders occurring simultaneously. The infusion of 1-desamino-8-d-arginine vasopressin (DDAVP) induced, in the propositus, a normalization of circulating levels of vWf, with a less pronounced enhancement of factor VIII:C. In the father, the response to DDAVP infusion of factor VIII/vWf complex was normal. In the mother, the time-course of factor VIII:C was characterized, after a peak at 30 min, by a progressive decrease until 2 hours after infusion, in contrast to vWf which appeared further increased at the same times. Therefore, the low factor VII:C/vWf:Ag ratio, already present before infusion, became significantly more pronounced 2 hours after DDAVP. Similar findings were observed in another obligate carrier of the family, in the propositus' sister and in 10 other haemophilia A carriers, belonging to different kindreds. In all patients, even when the basal factor VIII:C/vWf:Ag ratio was normal, two hours after DDAVP it decreased in agreement with the haemophilia A carrier state.