Background: Increased reactive oxygen species (ROS) have been implicated in the pathophysiology of chronic obstructive pulmonary disease (COPD). Objective: This study examined the effect of exogenous and endogenous oxidative stress on macrophage phagocytosis in patients with COPD. Methods: Monocyte-derived macrophages (MDM) were generated from non-smoker, smoker and COPD subjects, differentiated in either GM-CSF (G-Mϕ) or M-CSF (M-Mϕ). Alveolar macrophages were isolated from lung tissue or bronchoalveolar lavage. Macrophages were incubated +/- 200M H2O2 for 24 hours, then exposed to fluorescently-labelled H. influenzae or S. pneumoniae for 4 hours, after which phagocytosis, mitochondrial ROS (mROS), and mitochondrial membrane potential (m) were measured. Results: Phagocytosis of bacteria was significantly decreased in both G-Mϕ and M-Mϕ from COPD patients, compared to non-smoker controls. In non-smokers and smokers, bacterial phagocytosis did not alter mROS or m, however in COPD, phagocytosis increased early mROS and decreased m in both G-Mϕ and M-Mϕ. Exogenous oxidative stress reduced phagocytosis in non-smoker and COPD alveolar macrophages, and non-smoker MDM, associated with reduced mROS production. Conclusion: COPD macrophages show defective phagocytosis, which is associated with altered mitochondrial function and an inability to regulate mROS production. Targeting mitochondrial dysfunction may restore the phagocytic defect in COPD.