Early ante-natal diagnosis of haemophilia A and the detection of female carriers is now possible in some cases by analysis of DNA. The diagnosis may be established with a 100 p. 100 reliability in subjects with large deletion by direct analysis, and in 40 p. 100 of haemophiliac families by linkage studies with the intra-genic polymorphism revealed by the restriction enzyme BcII. Intensive research indicates that this percentage will increase in the near future. In the meantime, indicative studies are possible in other families. They consist in studying extra-genic restriction polymorphism. Over 90 p. 100 of families with haemophilia A may benefit from these studies using the probes currently available. Recombination, although not yet described, remains possible, and therefore ante-natal diagnoses made by the extra-genic probe should be controlled by foetal blood sampling at the 20th week of pregnancy.