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Asthma and chronic obstructive pulmonary disease (COPD) are common chronic airway diseases. Overlap in the clinical features of these 2 diseases is observed in many cases, and thus, the concept of asthma–COPD overlap (ACO) has recently been proposed. However, the definition of ACO and the clinical significance remains to be determined.We evaluated the prevalence and risk of acute exacerbation in ACO among Korean COPD patients as defined by modified Spanish criteria and American Thoracic Society (ATS) Roundtable criteria.The prevalence of ACO was 47.7% (660/1383) by modified Spanish criteria and 1.9% (26/1383) by ATS Roundtable criteria. ACO, regardless of criteria, did not significant affect the exacerbation risk during at least 1-year follow-up period.Substantial discrepancies were found in the prevalence and outcome of ACO according to different diagnostic criteria, which would compromise implementation of ACO before the definition is established.