Objective To analyze the clinical features of overlap syndromes(OS),I.e.,chronic obstructive pulmonary disease(COPD)complicated with obstructive sleep apnea hypoventilation syndrome (OSAHS).MethodsFrequency of episodes of acute exacerbation,severity,quality of life,exercise endurance,complications and three-year survival rate were investigated in eight patients with OS and 10 patients with pure COPD.Results Both quality of life and exercise endurance in OS group were lower than those in COPD group(P＜0.05).Annual frequency of acute exacerbation episodes and severity were more and three-year survival rate was lower in OS group than those in COPD group(P＜0.05).Complications of hypertension,diabetes,erythrocythemia,respiratory failure and heart failure were all more frequent in OS group than those in COPD group.Among all the above clinical features,increased frequency of acute exacerbation episodes was the most significant.Conclusions OS is clinically more severe than pure COPD,so more attention should be paid to it.Survival for patients with OS can not extended by nocturnal oxygen therapy alone,which may be beneficial when combined with biphasic positive airway pressure(BiPAP)or continuous positive airway pressure(CPAP)ventilation.