Background: Although asthma and COPD have been accepted as different diseases, many elderly patients show clinical features of both asthma and COPD, so called "overlap syndrome". The purpose of our study was to examine the prevalence and clinical characteristics of the overlap syndrome (OS). Methods: We conducted a retrospective survey of patients above 40 years who had visited our hospital from January 2005 to June 2009 because of obstructive lung disease. Patients with structural lung disease were excluded and patients with short follow-up duration were excluded. Based on lung function profile, we classified subjects into COPD and OS groups, and analyzed their prevalence, demographics and lung function. Results: 2,105 subjects were included. 240 (11.4%) and 630 (29.9%) were found to have OS and COPD, respectively. The mean age was similar in both group (66.40 years vs 67.67 years, P=0.553) but age of symptom onset was earlier in OS group than COPD group (38.42 years vs 58.17 years, P＜0.001). OS group showed lower baseline postbronchodilator FEV1 than COPD group (50.88% vs 57.65%, P=0.04). After additional treatment with tiotropium, OS and COPD groups showed significant improvement of postbronchodilator FEV1 (5.50%, P=0.012; 7.68%, P=.0.000). Conclusion: Certain proportion of aged patients have overlap syndrome. Although their symptom develops earlier and baseline lung function is worse, intensive treatment can improve lung function.