Factors relevant to the successful application of adeno-associated virus (AAV) vectors for liver-directed gene therapy were evaluated. Vectors with different promoters driving expression of human α-1-antitrypsin (α-1AT) were injected into the portal circulation of immunodeficient mice. α-1AT expression was stable but dependent on the promoter. Southern analysis of liver DNA revealed approximately 0.1 to 2.0 provirus copies/diploid genome in presumed head-to-tail concatamers. In situ hybridization and immunohistochemical analysis revealed expression in approximately 5% of hepatocytes clustered in the pericentral region. These results support the use of AAV as a vector for diseases treatable by targeting of hepatocytes.