In this study, we reinvestigated a 20-year-old woman, the first cousin of two brothers with severe haemophilia A. This patient was previously assumed to be a carrier of haemophilia A due to her FVIII deficiency. We identified a novel FVIII gene mutation in the family and demonstrated that the FVIII deficiency in this female patient did not result from this gene mutation, but was linked to molecular defects in the von Willebrand factor gene.