Hemophilia A is caused by a deficiency of blood coagulation factor VIII (FVIII) and has been widely discussed as a candidate for gene therapy. While the natural canine model of hemophilia A has been valuable for the development of FVIII pharmaceutical products, the use of hemophiliac dogs for gene therapy studies has several limitations such as expense and the long canine generation time. The recent creation of two strains of FVIII-deficient mice provides the first small animal model of hemophilia A. Treatment of hemophiliac mice of both genotypes with potent, human FVIII-encoding adenoviral vectors resulted in expression of biologically active human FVIII at levels, which declined, but remained above the human therapeutic range for over 9 months. The duration of expression and FVIII plasma levels achieved were similar in both hemophiliac mouse strains. Treated mice readily survived tail clipping with minimal blood loss, thus showing phenotypic correction of murine hemophilia A by in vivo gene therapy.