Alpha-1-antitrypsin is a relatively common genetic deficiency that results in early emphysema. The liver as the natural source of most alpha-1-antitrypsin synthesis was the target organ selected for gene replacement therapy studies. Previous work used recombinant retroviral vectors that encode the human alpha-1-antitrypsin cDNA for ex uiuo and direct in uiuo transduction of hepatocytes in dogs and rodents. This approach led to low levels of the human protein in the serum of recipients. In this study, recombinant adenoviral vectors that express the human alpha-1-antitrypsin cDNA under the transcriptional control of the phosphoglycerate kinase (PGK) or RSV-LTR promoters have been constructed and used for the direct transduction of mouse hepatocytes in uiuo. The animals transduced with the recombinant adenoviral vectors had therapeutic serum levels of human alpha-1-antitrypsin of up to 700 pg/mL. Thus, adenovirus-mediated gene transfer of the hAAT cDNA into the liver was able to produce therapeutic serum concentrations of protein. (HEPATOLOGY 1995;21:815-819.) Alpha-1-antitrypsin (AAT) deficiency results in early chronic obstructive pulmonary emphysema or infantile liver chirrosis. AAT is a serum serine protease inhibitor whose most important function is to balance the alveolar neutrophil elastase activity in the lung. Most of the protein is made in the liver, although trace quantities of unknown significance are made in macrophages and neutrophils.' More than 75 different mutant alleles have been described; the most common normal allele Abbreviations: AAT, alpha-1-antitrypsin; hAAT, human AAT; cDNA, complementary DNA PGK, phosphoglycerate kinase; RSV-LTR, Rous sarcoma virus-long terminal repeat.