A 6-year-old girl, daughter of a male patient with moderate hemophilia A (factor VIII 3%), was referred to our Center because she also had very low levels of factor VIII (4%). The proband's brother has mild hemophilia A (7%); the mother (29%) is a possible carrier, no other case of hemophilia A being reported in her family. Hence, the factor VIII deficiency found in the girl is consistent either with a carrier state with extreme lyonization in favour of the hemophilic gene or with homozygosity for the hemophilia gene. To distinguish these possibilities, we studied the segregation of three restriction fragment length polymorphisms (RFLPs) linked to the factor VIII gene in the 4 members of the family. Employing one intragenic (FVIII-BclI) and two extragenic (St14-TaqI and Dx13-BglII) RFLPs, we showed that the proband has inherited from both parents the defective gene, being therefore homozygous for the hemophilia gene.