Cloned factor VIII deoxyribose nucleic acid (DNA) sequences were used as probes in the prenatal diagnosis of haemophilia A. Fetal DNA from cultured amniotic fluid cells was examined for a DNA polymorphism within the factor VIII gene which marked the haemophilia A gene in the pregnant obligate carrier. The fetus was predicted to be an affected male, and the diagnosis of haemophilia A was confirmed both in utero and after termination of the pregnancy.