Chronic obstructive pulmonary disease (COPD) and obstructive sleep apnoea (OSA) syndrome are both highly prevalent, affecting at least 10% of the general adult population, and each has been independently associated with an increased risk of cardiovascular disease. The presence of both disorders together, commonly referred to as the overlap syndrome, is also highly prevalent, although various clinical and pathophysiological factors associated with COPD may increase or decrease the likelihood of OSA. Lung hyperinflation reduces the likelihood of obstructive apnoea, whereas right heart failure increases the likelihood as a result of rostral fluid shift causing upper airway narrowing in the supine position while asleep. Furthermore, upper airway inflammation associated with OSA may aggravate lower airway inflammation in COPD. The proposed mechanisms of cardiovascular disease in each disorder are similar and include systemic inflammation, oxidative stress, and sympathetic excitation. Thus, one could expect that the prevalence of co-morbid cardiovascular disease would be higher in the overlap syndrome but, with the exception of pulmonary hypertension, there are few published reports that have explored this aspect in depth. Hypoxia is more pronounced in patients with the overlap syndrome, especially during sleep, which is likely to be the principal factor accounting for the recognised higher prevalence of pulmonary hypertension in these patients. Cardiac sympathetic activity is increased in patients with the overlap syndrome when compared to each disorder alone, but echocardiographic evidence of left ventricular strain is no greater in overlap patients when compared to COPD alone. While survival might be expected to be worse in overlap patients, recent evidence surprisingly indicates that the incremental contribution of lung function to mortality diminishes with increasing severity of OSA. Identification of co-morbid OSA in patients with COPD has practical clinical significance as appropriate positive airway pressure therapy in COPD patients with co-existing OSA is associated with improved morbidity and mortality.