Hemophilia A is an X-linked hemorrhagic disorder caused by heterogeneous mutations in the factor VIII gene. Recently, it was reported that approximately 50% of the cases of severe hemophilia A may be caused by a common inversion in the factor VIII gene. In this study, we analyzed 33 Japanese patients with severe hemophilia A for the presence of this inversion mutation using the non-RI Southern blotting, and detected inversion mutations of the factor VIII gene in 12 patients (36.4%). We also showed that the inversion analysis of the factor VIII gene was useful for carrier detection and prenatal diagnosis in a hemophilia A family, which we had not been able to diagnose by the analysis of restriction fragment length polymorphisms (RFLPs) of the factor VIII gene. The detection of inversion mutations in the factor VIII gene using non-RI Southern blotting analysis appears to be very useful for the genetic counseling for severe hemophilia A.