Sporadic occurrences of X-linked disorders can give insights into mutagenesis in man. In a case of sporadic hemophilia, associated with a partial deletion of the factor VIII gene, an unexpected inheritance pattern of gene rearrangements was observed. The factor VIII gene was found to be partially duplicated in the hemophiliac's mother. A pedigree analysis indicates that the mother has contributed both aberrant genes as well as the normal gene to her offspring. One simple model for the evolution of the deletion in this family is that the duplication is the precursor to the deletion.