This is a pragmatic trial designed to test if a treatment, losartan, may decrease the progression of Chronic Obstructive Pulmonary Disease (COPD). Losartan is an angiotensin receptor blocking agent, commonly used as an antihypertensive agent, which has been shown to alter cardiac remodeling after myocardial infarction and renovascular remodeling in diabetes-mellitus. In this trial participants with mild to severe COPD, with Computed Tomography (CT) evidence of emphysema (5-35% of voxels with < -950 Hounsfield Units), will be randomly assigned to receive 100mg/day of losartan or placebo for 48 weeks. The primary outcome measure will be the rate of progression of emphysema, quantified as the percent of lung voxels with a density less than -950 HU as measured by High Resolution CT (HRCT, from baseline to 48 weeks.