Despite the high prevalence of chronic obstructive pulmonary disease (COPD), there continues to be a large gap in our understanding of disease pathogenesis and mechanisms accounting for large variability in disease phenotype. Cigarette smoking is the principal cause of COPD, but only approximately 15% of adults with substantial tobacco exposure develop clinical COPD. Besides, bacterial colonization or infection is also considered as an important factor in COPD. There are very limited data from microbiome studies that suggest that respiratory and gastrointestinal microbiota may be involved in the pathogenesis of COPD. However, the cross-talk between between lung and gut microbiome, and their relationship with various clinical phenotypes of COPD. Here, we conducted 16S rRNA-based pyrosequencing to evaluate the link between the lung-gut axis and the clinical phenotypes of COPD, such as lung function, emphysema, symptoms, exacerbations, inflammation levels and metabolic features.