The diagnosis of haemophilia A and the identification of carriers has greatly improved with knowledge of the structure of the gene for factor VIII. This has permitted the defect to be tracked in families by the study of restriction fragment length polymorphisms (RFLPs), irrespective of the nature of the molecular defect. However, this approach is time-consuming and the information yielded falls away as more polymorphisms are added. Within the factor VIII gene lies another source of polymorphism, a dinucleotide repeat sequence of varying length known as (CA)n. Conventional mapping localised this (CA)n repeat to intron 13. The polymerase chain reaction, used to examine (CA)n variability in genomic DNA from 25 males and 67 females, revealed eight allelic bands between n = 16 and n = 24. 91% of females were heterozygous for this repeat, and family studies showed X-linked mendelian inheritance with allelic frequencies ranging from 1% to 45%. The intron 13 (CA)n repeat is tightly linked with established RFLPs and tracks with haemophilia A in family studies. The analysis requires DNA from other family members, and relatives of sporadic cases of haemophilia A are only amenable to exclusion. Nonetheless, this intron 13 (CA)n repeat provides the most highly informative marker so far available for factor VIII gene tracking studies in haemophilia A kindreds and a result can be available within a day.