Factor VIII (FVIII) gene rearrangements between the intron 22 F8A sequence in the FVIII gene and either of the two homologous F8A sequences 500 kilobases telomeric to the FVIII gene have recently been found to be responsible for the severe hemophilia A phenotype. We studied 27 patients with severe hemophilia A and 19 with moderate and mild hemophilia, and found FVIII gene rearrangement in 12 patients with severe hemophilia A and none in the patients with moderate or mild disease. Nine of the rearrangements were with the distal telomeric F8A sequence, two were with the proximal sequence, and one had variant distal rearrangement with loss of the FVIII intron 22 F8A band. Two patients with FVIII gene rearrangement had high responding inhibitors, contrary to one previous study suggesting that the presence of a FVIII gene rearrangement is correlated with the absence of inhibitor development. Carrier detection was performed in 17 female relatives, at risk of being carriers, from eight kindreds; 13 were carriers, being heterozygous for the normal and rearranged alleles. The rearrangement assay is particularly useful for carrier determination in families with sporadic cases of hemophilia not helped by linkage analysis with restriction fragment-length polymorphism or intragenic dinucleotide repeat analysis. In all five families with rearrangements and sporadic hemophilia, the mothers of all index patients were found to be carriers.