Rationale: We have developed a novel oxygen adherence monitor that objectively measures patient use of long-term oxygen therapy. The monitor attaches to the oxygen source and detects whether or not the patient is wearing the nasal cannula. Objective: The study's purpose was to validate the monitor's performance in patients with chronic obstructive pulmonary disease during wakefulness and sleep. Methods: Ten adult males with stable chronic obstructive pulmonary disease (mean ± SD FEV1 37.7 ± 14.9% of predicted) on long-term continuous oxygen therapy were tested in a sleep laboratory over a 12–13 hour period that included an overnight polysomnogram. Measurements: The monitor's measurements were obtained at 4-minute intervals and compared to actual oxygen use determined by review of time-synchronized video recordings. Main results: The monitor made 1504/1888 (79.7%) correct detections (unprocessed data) across all participants: 957/1,118 (85.6%) correct detections during wakefulness and 546/770 (70.9%) during sleep. All errors were false negatives, i.e., the monitor failed to detect that the participant was actually wearing the cannula. Application of a majority-vote filter to the raw data improved overall detection accuracy to 84.9%. Conclusions: The results demonstrate the monitor's ability to objectively measure whether or not men with chronic obstructive pulmonary disease are receiving their oxygen treatment. The ability to objectively measure oxygen delivery, rather than oxygen expended, may help improve the management of patients on long-term oxygen therapy.