The association between alpha 1-antitrypsin deficiency and heritable emphysema was discovered in 1963. Subsequent epidemiologic evidence suggested that a serum alpha 1-antitrypsin level of 11 mumol/L (about 80 mg/dL by the still-used "commercial standard"), which is about 35% of the normal level, represents a "threshold" value, below which the risk of developing emphysema is increased and above which the emphysema risk is not increased. Recently, the ability to isolate and purify the alpha 1-antitrypsin protein from human blood has made "specific" augmentation therapy possible. Intravenous infusion of alpha 1-antitrypsin raises serum and alveolar levels above the putative thresholds, but clinical efficacy (i.e., decreased rate of decline in lung function and/or improved survival) remains presumptive. Based on available evidence, the American Thoracic Society recommends augmentation therapy for individuals with both a documented severe deficiency of alpha 1-antitrypsin and fixed airflow obstruction.