Persistent therapeutic levels of human factor VIII (hFVIII) would signify a major advance in the treatment of hemophilia A. Here we report sustained expression of hFVIII in immunocompetent mice using recombinant adeno-associated virus (rAAV) vectors. AAV can stably transduce liver cells, the target tissue for efficient hFVIII production. Because of rAAV packaging constraints, we tested 2 constructs using small regulatory elements designed for liver-specific transgene expression linked to B-domain-deleted hFVIII (BDD-hFVIII) cDNA. More than 10(12)/mL rAAV/BDD-hFVIII virion particles were generated using a transfection scheme that eliminates adenovirus. Coatest and APTT assays confirmed the production of functional BDD-hFVIII protein after transduction of 293 and HepG2 cells. In vivo experiments were performed in C57BL/6 and NOD/scid mice receiving 10(10-11) rAAV/hFVIII particles via portal vein injection. All C57BL/6 mice tested developed anti-hFVIII antibody. In contrast, NOD/scid mice expressed hFVIII reaching 27% of normal human plasma levels. As expected, we could not detect hFVIII antigen from plasma samples isolated from control animals receiving equivalent doses of rAAV expressing enhanced green fluorescent protein (EGFP). Transgene mRNA expression was detected primarily in the liver and histologic analysis of the liver revealed no pathologic abnormalities. These results demonstrate a promising approach for treatment of hemophilia A. (Blood. 2000;95:1594-1599)