Extensive research, stimulated by the recognition of an association between alpha 1-antitrypsin (alpha 1-AT) deficiency and emphysema, has greatly advanced our understanding of emphysema in general. In this article, we review the literature concerning the basic defect, inheritance, pathogenesis of lung disease, clinical, physiologic, and roentgenographic findings in patients with severe (Pi SZ) deficiency of alpha 1-AT. Data obtained in relatives with alpha 1-AT deficiency, who have not been seen by a physician, should more truly reflect the risk of having lung disease. These studies confirm the increased risk of developing lung disease in cigarette smokers. The implications of the finding that subjects with Pi SZ are probably not at an increased risk of lung disease is discussed with regards to replacement therapy. The natural history of unselected subjects with alpha 1-AT deficiency remains unknown.