Deficiency of the A subunit of coagulation factor XIII causes a severe bleeding disorder requiring life long replacement therapy. The mutations causing A subunit deficiency appear to be very heterogeneous, and it is impractical to identify each mutation before genetic counselling or prenatal diagnosis can be attempted. In this study we have shown that a highly polymorphic short tandem repeat element, HUMF13A01 (AAAG)n that occurs in the 5' flanking sequence of the factor XIII A subunit gene, can be used to follow the segregation of deficiency causing mutations. We studied 6 families with factor XIII A subunit deficiency from 5 different ethnic groups. All parents were heterozygous for the repetitive element and therefore all the families were informative. The linked polymorphism was used to carry out the first prenatal diagnosis of factor XIII A subunit deficiency. The analysis of this polymorphism by the polymerase chain reaction is rapid, reliable, requires little DNA and is ideal for the genetic analysis of factor XIII A subunit deficiency.