alpha 1-Antitrypsin (AAT) is a polymorphic protein with many variants collectively known as the Pi system. The most common alleles are the M, S and Z, which are co-dominantly inherited. Infants with PiZZ have approximately 16% of the normal AAT serum concentration. alpha 1-Antitrypsin deficiency (AATD) is an inborn error of metabolism which is principally associated with liver disease in children and emphysema in young adulthood. Individuals with AATD produce an abnormal protein which accumulates in the liver, resulting in decreased serum levels. Affected individuals cannot protect their lungs from digestion by elastase. Smoking is a significant risk factor for the early development of emphysema. Prolastin, human alpha 1-protease inhibitor, is now available as replacement therapy. Weekly intravenous administration, with the goal of maintaining the serum AAT greater than 80 mg/dl, appears to arrest pulmonary damage. Its effect on liver disease is unknown at this time. A recombinant alpha 1-protease inhibitor is being tested in aerosol form with promising early results.