The carrier status of women in five families with an isolated haemophilia male was assessed by pedigree analysis, coagulation factor assays and DNA markers. In three families, ten women could be given very low risks of being carriers based on DNA analysis. In two of the families the DNA markers identified the mutation as originating in either the maternal or maternal grandfather's germ cell. Combined DNA and coagulation data suggested that the affected male in a third family was a de novo mutation. DNA analysis of the affected male in another family identified a large deletion of the F8 gene which was present in his mother and three sisters, suggesting that the grandmother was a carrier. A combination of coagulation factor data and DNA marker assessment can determine the carrier status of the majority of females in families with isolated affected haemophilia A males.