Some bacterial infections may worsen COPD exacerbations. Current studies are examining if the macrolide antibiotic azithromycin may be beneficial at reducing the frequency and/or severity of COPD exacerbations. Azithromycin also has anti-inflammatory properties that may reduce the severity of COPD exacerbations by inhibiting the matrix metalloprotease (MMP)-catalyzed breakdown of collagen and the subsequent generation of proline-glycine-proline (PGP), a substance produced in response to collagen breakdown. An increase in PGP levels may indicate an increase in inflammation, which can worsen COPD symptoms. NHLBI's COPD Network Macrolide study includes people with COPD who were randomly assigned to receive either azithromycin or placebo for 1 year. For this current study, researchers will examine the Macrolide participants' previously collected blood samples, sputum samples, and study data, including information on COPD exacerbations and azithromycin effects. The purpose of this study is to examine the anti-inflammatory properties of azithromycin in people with COPD.