<b><i>Background:</i></b> The occurrence of both chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) in an individual patient has been described as ‘overlap syndrome', which has been associated with poor prognosis. Little is known about the possible predictors of the overlap syndrome and its association with comorbidities contributing to impaired outcome. <b><i>Objectives:</i></b> This study aimed to evaluate the prevalence and possible predictors of the overlap syndrome and its association with comorbidities in a cohort of COPD patients. <b><i>Methods:</i></b> Individuals with COPD (GOLD stages I-IV, risk groups A-D) were recruited from outpatient clinics. Information on age, gender, body mass index (BMI), smoking status, Epworth sleepiness scale (ESS), COPD assessment test, comorbidities, medications and exacerbations in the past year was collected and a spirometry was performed. Participants underwent a nocturnal polygraphy using the ApneaLink™ device at home. An apnea-hypopnea index (AHI) >10 per hour was considered to indicate OSA. <b><i>Results:</i></b> We enrolled 177 COPD patients (112 men) with a mean age of 64 years (range 42-90), of whom 35 (20%) had an ESS score above 10. During nocturnal polygraphy, 33 patients (19%) had evidence of OSA. In multivariate analysis, BMI and pack years were positively associated with AHI, independent of other significant AHI determinants from univariate analysis. Arterial hypertension and diabetes were more common in patients with the overlap syndrome. <b><i>Conclusions:</i></b> Almost 20% of COPD patients also have OSA. BMI and smoking history seem to be predictors of the overlap syndrome, and these patients may be more often affected by hypertension and diabetes.