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COPD and OSA are both highly prevalent, which implies that both disorders occurring together (overlap syndrome) is likely to be common based on chance association alone. However, different clinical COPD phenotypes influence the likelihood of coexisting OSA in that the increased lung volumes and low BMI associated with the predominant emphysema phenotype protects against OSA, whereas the higher likelihood of peripheral edema and increased BMI associated with the predominant chronic bronchitis phenotype promotes OSA. Both COPD and OSA are associated with similar physiological and molecular consequences, such as hypoxia and systemic inflammation, that contribute to cardiovascular and other comorbidities, and pulmonary hypertension is highly prevalent in patients with the overlap syndrome. However, there have been few published reports that have evaluated systemic inflammation and other cardiovascular comorbidities in patients with overlap syndrome. The diagnosis of OSA in patients with COPD requires awareness of relevant clinical features, and screening questionnaires may help identify suitable patients for further overnight study. The recognition of coexisting OSA in patients with COPD has important clinical relevance, as the management of patients with overlap syndrome is different from the management of COPD alone, and the survival of patients with overlap syndrome that is not treated with nocturnal positive airway pressure is significantly inferior to that of patients with overlap syndrome that is appropriately treated.