A family survey of mild hemophilia A has been carried out during the last 25 years. Presently these families contain 27 living affected males or approximately twice the number of males with severe form of hemophilia A in Iceland. The aim of the study was to define the phenotype in three families with mild haemophilia A and to determine RFLP (Restriction Fragment Length Polymorphism) polymorphisms, which could support a hypothesis of a common progenitor of the families. Family survey was made and index cases were identified in and outside hospitals and a survey for symptoms and signs of bleeding in family members and sampling for coagulation and RFLP studies were mostly carried out in the field. Family members with and without symptoms of bleeding were selected for investigation and normal spouses and unrelated individuals were investigated for control. Bleeding time, factor VIII activity, quantification of factor VIII Ag, von Wil-lebrand factor Ag and vWF ristocetin assay. Typing of RFLP polymorphisms for genetic homogeneity. Bleeding manifestations are present in both sexes in the three families although more frequent and more severe in the males. Prolonged bleeding time in 6 of 7 affected members in one of the families and subnormal levels in von Willebrand related assays point to an additional abnormality which has not been explained. The level of factor VIII activity is between 10-20% in most affected males whereas 35-60% is found approximately in 2/3 of females carriers and in 1/3 of them factor VIII activity is within the normal range. Major bleeding episodes in family members are associated with accidents, surgical procedures menorrhagia and in some cases provoked by diseases such as intestinal ulceration, kidney stones and abnormalities of vasculature. RFLP polymorphisms were detected, which are common to the families and they support hypothesis of a common progenitor. The genetic studies show relatively high prevalence of this type of mild haemophilia A, which seems to have been present for at least 6-8 generations in Iceland. It is suggested that screening for this mild haemophilia A gene by molecular genetic method would be of clinical value now, when it's mutation has been detected. Transmission of mild haemophilia A through 6-9 generations is demonstrated by the study. Founder effect is confirmed by studies of RFLP polymorphisms. The mild haemophilia A type described is the most prevalent of haemophilia A types in Iceland (population 265,000, 1993).