We have characterized an unusual duplication of exon 13 within the factor VIII gene in a patient with a mild form of hemophilia A. This duplication was the result of a nonhomologous breakage and reunion event of two misaligned wild-type chromosomes. Sequence analysis of the breakpoint region revealed the presence of AT-rich sequences and possible topoisomerase I sites, whose involvement in several cases of illegitimate recombination has been postulated.