The purpose of this research was to describe the frequency and characteristics of the overlap syndrome among stable COPD patients (stage 2 to 4 according to GOLD). Material and method: We studied 46 patients with stable COPD recruited from the outpatient clinic of the CHU of Liege from May 2013 to April 2014. Definition of the overlap syndrome was based on the coexistence of post-bronchodilation FEV1/FVC < 70% and, either, an asthmatic history before the age of 40, or, at least, two functional and immune-inflammatory asthmatic traits : 1) significant FEV1 reversibility to inhaled bronchodilator (FEVI change >/= 200 ml and >/= 12% after bronchodilation), 2) eosinophilic inflammation : sputum eosinophils ≥ 3%,blood eosinophils ≥ 400/μl, or FENO ≥ 45 ppb, 3) clinical history of airway allergy, or total serum IgE ≥ 113 KU/l, or RAST ≥ 0,35 KU/l against major aeroallergens. 37% patients had the COPD-asthma overlap syndrome and this group had a higher CAT score reflecting more severe symptoms (24,6 ± 8,1 vs 19,4 ± 8, p < 0,05) despite similar level of airway obstruction. The transfer coefficient DLCO/VA was preserved in the overlap group (97 ± 24%), but altered in the pure COPD group (80 ± 20%), p < 0,05. Approximately one third of COPD patients present with the overlap syndrome and they are more symptomatic without any evidence of more severe airway obstruction.