The Long-term Oxygen Treatment Trial (LOTT) is sponsored by the National Heart Lung and Blood Institute (NHLBI).  LOTT is a randomized controlled clinical trial to assess the efficacy of around-the-clock, supplemental oxygen therapy in patients with chronic obstructive pulmonary disease (COPD) and moderately severe hypoxemia.   LOTT is registered on ClinicalTrials.gov (NCT00692198).