Abstract: Objective To investigate effects of smoking on exhaled nitric oxide in patients with chronic obstructive pulmonarydiseases (COPD) or asthma COPD overlap syndrome (ACOS). Methods A total of 105 patients with COPD, 79 patients with ACOS and 172 healthy people were enrolled in this study. Clinicaldata, exhaled nitric oxide (FeNO) measurements and pulmonary function test results were retrospectively analyzed. Results The level of FeNO in non-smoking patients with COPD was significantly higher than smokers with COPD (29.46±8.12) ppb vs (25.07±9.78) ppb, P< 0.05, and the level of FeNO in non-smoking patients with ACOS was significantly higher than smokers with ACOS (42.50±15.53) ppb vs (34.53±18.11) ppb, P< 0.05, while the FeNO level in healthy smokers was (16.57±4.19) ppb and (17.01±4.17) ppb in non-smoking residents with no statistically significantdifference. The FeNO values in smoking patients with ACOS were negatively related to the smoking index (r=-0.42, P=0.015). Conclusion Smoking candecrease FeNO value in patients with COPD or AOCS. In patients with ACOS group, the higher the smoking index is, the lower the FeNO value becomes.