RATIONALE: Patients with chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) (overlap syndrome) are more likely to develop pulmonary hypertension than patients with either condition alone. OBJECTIVES: To assess the relation of overlap syndrome to mortality and first-time hospitalization because of COPD exacerbation and the effect of continuous positive airway pressure (CPAP) on these major outcomes. METHODS: We included 228 patients with overlap syndrome treated with CPAP, 213 patients with overlap syndrome not treated with CPAP, and 210 patients with COPD without OSA. All were free of heart failure, myocardial infarction, or stroke. Median follow-up was 9.4 years (range, 3.3-12.7). End points were all-cause mortality and first-time COPD exacerbation leading to hospitalization. MEASUREMENTS AND MAIN RESULTS: After adjustment for age, sex, body mass index, smoking status, alcohol consumption, comorbidities, severity of COPD, apnea-hypopnea index, and daytime sleepiness, patients with overlap syndrome not treated with CPAP had a higher mortality (relative risk, 1.79; 95% confidence interval, 1.16-2.77) and were more likely to suffer a severe COPD exacerbation leading to hospitalization (relative risk, 1.70; 95% confidence interval, 1.21-2.38) versus the COPD-only group. Patients with overlap syndrome treated with CPAP had no increased risk for either outcome compared with patients with COPD-only. CONCLUSIONS: The overlap syndrome is associated with an increased risk of death and hospitalization because of COPD exacerbation. CPAP treatment was associated with improved survival and decreased hospitalizations in patients with overlap syndrome.