Although certain environmental agents (e.g., cigarette smoking) are known to be causally related to chronic obstructive pulmonary disease (COPD), differential response to their deleterious effects suggests the importance of constitutional (host) factors. The voluminous literature on the familial occurrence of COPD as well as the association between genetically determined serum alpha 1-antitrypsin (alpha 1-at) deficiency and COPD, however, reveals many aspects yet to be clarified. Studies of alpha 1-at indicate that neither its nature nor its relationship to COPD is simple. Moreover, other familial factors are likely involved. To obtain a better understanding of the etiology and pathogenesis of COPD, both genetic and environmental factors are being examined in a multifaceted investigation. The preliminary observations are summarized. COPD patients have a higher frequency of Pi variant phenotypes than those without lung disease. Among other subjects, both cigarette smoking and Pi variant phenotypes are associated with increased pulmonary function abnormality. Finally, there is familial aggregation of pulmonary impairment that cannot be explained entirely by Pi type or smoking.