The Chronic Obstructive Pulmonary Disease Fish Oil (COD-Fish) Pilot Trial will enroll 40-45 participants with COPD. These participants will be randomized to receive omega-3 polyunsaturated fatty acid supplementation (PUFA) or placebo (containing corn oil). All participants will perform a number of noninvasive tests (including flow-mediated dilation (FMD) and pulmonary function testing) at study entry as well as every two months for a total of six months. Thirty participants who agree to it will undergo a separate procedure to collect cells from inside a forearm vein for analysis to be compared to 30 controls. The main purpose of this trial is to examine the effect of PUFA on endothelial function as measured by change in FMD in patients with COPD. We hypothesize that in subjects with COPD, treatment with PUFA will increase FMD.