The CHAIN study identified 125 of 831 (15%) patients diagnosed with COPD with characteristic manifestations of asthma, which were considered as ACOS.It was considered that if patients were earlier diagnosed with asthma or positive bronchial diastolic test (BDR), defined as >400cc and 15%; blood eosinophils >5%,IgE >100/mL; both airway diastolic tests tested separately were positive (>200cc and 12%), both of which met the diagnostic criteria for ACOS. The concept of ACOS is not new in the respiratory field. The classification and relevance of asthma and COPD have been debated for several years. One of the “Dutch hypotheses” preferred to create a new classification, which was published in Chest in 2004. In addition, several other ACOS ideas have been published, and even some organizations have recommended the development of diagnostic criteria. The National Asthma Education and Prevention Program Expert Panel Report defines asthma as “a complex disease characterized by diverse and recurrent symptoms, airflow limitation, airway hyperreactivity, and an underlying inflammatory response”. COPD is defined in the GOLD guidelines as “a common preventable and treatable disease characterized by persistent airflow limitation, which is usually progressive and associated with exacerbation of the chronic inflammatory response”. Asthma and COPD are both inflammatory diseases characterized by airflow limitation. Therefore, the confusion between the two, and even the concept of ACOS, is well documented. Asthma often develops at a young age, while COPD often develops after the age of 40. Asthma has a diverse symptomatology and course, whereas COPD presents with a stable, progressive clinical decline. Patients with asthma have BDR whereas COPD does not. Most guidelines separate asthma from COPD on this basis. However, in practice, it is not uncommon for middle-aged and middle-aged patients with asthma and COPD to be clinically uncommon. Asthmatics are at risk of airway remodeling, fixed deficits in lung function, and 50% of COPD patients also develop at least one clearly significant BDR over time.In conclusion, age of onset, evolution of disease course, and BDR are helpful in the differential diagnosis, but not absolute. In patients with asthma, ICSs (inhaled glucocorticoids) are an important part of the treatment, and according to GOLD guidelines, ICSs are only used in specific subgroups of COPD (FEV1<50% or recurrent acute exacerbations), in order to reduce their acute exacerbations. However, COPD patients on long-term ICS are at risk of developing pneumonia and osteoporosis. Therefore, although the two have similar treatment approaches, the treatment outcomes are different, and it is important to clarify the cause and treat it precisely. About 10-20% of patients with asthma have COPD, and 10-20% of patients with COPD also have asthma. Although cytology, genetics, lung function and histological tests do not provide a clear identification of asthma, COPD or ACO, clinical features may suggest the presence of ACOS to the internist. The data show that COPD patients with high eosinophil counts in sputum or blood have improved FEV1 and fewer exacerbations with ICS therapy, leading to a recommendation to add ACO to the 2013 GOLD regimen. However, the use of LABA (long-acting β2-agonist) in COPD patients without ICS increased mortality in asthmatic patients. In other words, single LABA therapy is a risk for ACO patients.