The co-existence of chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA), termed the overlap syndrome (OVS), is associated with adverse outcomes that may be reversed with treatment. However, diagnosis is limited by the apparent need for in-laboratory polysomnography (PSG). WatchPAT is a portable diagnostic device that is validated for the diagnosis of OSA that might represent an attractive tool for the diagnosis of OVS. Subjects with established COPD were recruited from a general population. Subjects underwent PSG and simultaneous recording with WatchPAT. Pulmonary function testing and questionnaires were also performed. A total of 36 subjects were recruited and valid data was obtained on 33 (age 63 +/- 7, BMI 28 +/- 7, 61% male, FEV1 56 +/- 20% predicted). There was no significant difference in the apnea-hypopnea index (AHI) between PSG and WatchPAT (19 +/- 20 versus 20 +/- 15 events/h; mean difference 2(-2, 5) events/h; p = 0.381). The AHI was not significantly different in rapid eye movement (REM) and non-rapid eye movement (NREM) determined by PSG versus REM and NREM determined by WatchPAT. WatchPAT slightly overestimated total and REM sleep time, and sleep efficiency. The sensitivity of WatchPAT at an AHI cut-off of >= 5, >= 15, and >= 30 events/h for corresponding PSG AHI cut-offs was 95.8, 92.3, and 88.9, respectively; specificity was 55, 65.0, and 95.8, respectively. WatchPAT is able to determine OSA reliably in patients with COPD. The availability of this additional diagnostic modality may lead to improved detection of OVS, which may in turn lead to improved outcomes for a group of COPD patients at high risk of poor outcomes.