Considerable attention has been focused upon alpha-1-antitrypsin deficiency because of the insights into the pathogenesis of human pulmonary emphysema that may derive from study of deficient subjects, and because of evolving thereapeutic strategies that may slow the progression of lung disease in affected persons. We have applied an automated immunoassay for alpha-1-antitrypsin to plasma samples from 20,000 blood donors. Seven PI Z antitrypsin-deficient persons were identified and confirmed; this is more than twice the number predicted from previous estimates of the Z allele frequency in the St. Louis area. Five of the subjects were further evaluated. We anticipated that this assay, if utilized to screen large populations, could identify many alpha-1-antitrypsin-deficient persons for study of the natural history of lung and liver disease associated with the deficiency. These subjects would be potential candidates for early intervention strategies to prevent the development of lung disease. The surprisingly high prevalence of deficient persons indicates that direct screening is the best method for prevalence estimation of genetic disorders.