Patients with alpha 1-antitrypsin (AAT) deficiency, like those with asthma and chronic obstructive pulmonary disease, usually present with dyspnea, wheeze, and cough. The similarity in presentation and unfamiliarity among clinicians with AAT deficiency account for much of the delay in diagnosis. Normally, AAT inhibits serine proteases, which cause alveolar destruction, and alters the function of cells that release mediators of inflammation. Diagnostic findings suggesting deficiency include irreversible airflow obstruction, a decreased diffusing capacity of the lung for carbon monoxide, bibasilar bullous disease on chest films, and a low serum level of AAT. Asthma is usually diagnosed on the basis of clinical findings and response to inhaled beta agonists. The presence of inflammation is believed to be necessary for development of clinically significant asthma. Inflammation added to a deficiency of antiprotease inhibitor activity significantly worsens bronchial hyperreactivity. This is only one mechanism by which AAT deficiency may potentiate allergic and bronchospastic responses. The prevalence of bronchial asthma in patients with AAT deficiency is unknown. Studies by the National Institutes of Health regarding the natural history of AAT deficiency and its response to therapy are under way. Perhaps more will be discovered about the relationship between the disorder and bronchial asthma.