The marked variability in the development of chronic obstructive pulmonary disease (COPD) in response to cigarette smoking has been known for decades, but severe alpha 1-antitrypsin deficiency (PI Z) remains the only proven genetic risk factor for COPD. With cigarette smoking, PI Z subjects tend to develop more severe pulmonary impairment at an earlier age than non-smoking PI Z individuals. However, PI Z individuals exhibit wide variability in pulmonary function impairment, even among individuals with similar smoking histories. Therefore, other genes and environmental exposures are also likely involved. The role of heterozygosity for the Z allele as a risk factor for COPD remains controversial, but accumulating evidence suggests that at least some PI MZ individuals are at increased risk of developing airflow obstruction. In individuals without alpha 1-antitrypsin deficiency, familial aggregation of COPD has been reported in several studies. To study novel genetic determinants of COPD, our research group enrolled 44 severe, early-onset COPD probands (FEV1 < 40%, age < 53 yrs, non-PI Z) and 266 of their relatives. A marked female predominance was noted among the early-onset COPD probands. In addition, increased risk to current or ex-smoking first-degree relatives of early-onset COPD probands for reduced FEV1, chronic bronchitis and spirometric bronchodilator responsiveness has been demonstrated. These data strongly support the genetic basis for the development of COPD and the potential for gene-by-environment interaction. A variety of studies have examined candidate gene loci with association studies, comparing the distribution of variants in genes hypothesized to be involved in the development of COPD in COPD patients and control subjects. For most genetic loci which have been tested, there have been inconsistent results. Genetic heterogeneity could contribute to difficulty in replicating associations between studies. In addition, case-control association studies are susceptible to supporting associations based purely on population stratification, which can result from incomplete matching between cases and controls--including differences in ethnicity. No association studies in COPD have been reported which used family-based controls, a study design which is immune to such population stratification effects. More importantly, no linkage studies have been published in COPD to identify regions of the genome which are likely to contain COPD susceptibility genes--regions in which association studies are likely to be more productive.