Objective To discuss the influence of smoking on fractional exhaled nitric oxide (FeNO) expression in pa-tients with chronic airway inflammation. Methods According to the clinical history and characteristics of lung function, 206 patients were divided into asthma and chronic obstructive pulmonary disease (COPD) overlap syndrome (ACOS) group (n=37), COPD group (n=124) and asthma group (n=45). Meanwhile, 40 people accepted healthy physical examination were used as the control group (n=40). Then persons were sub-divided into smokers or nonsmokers according to the situation of smoking. The FeNO value and pulmonary function index were compared between the four groups, and the FeNO value was compared between smokers and nonsmokers respectively. The smoking index and FeNO value of smokers were measured for correlation analysis. Results (1)The FeNO values were significantly higher in ACOS group and asthma group than those of COPD group and the control group (32.6±9.9 and 37.6±10.9 vs 18.7±9.8 and 14.4±4.3,F=68.082,P<0.05). However, there were no significant differences in FeNO value between ACOS group and asthma group, and between COPD group and the control group. (2) The FEV1/FVC was significantly lower in ACOS group, COPD group and asthma group than that of control group, while the FEV1/FVC was significantly lower in ACOS group and COPD group than that of asthma group(P<0.05). There was no significant difference in FEV1% between ACOS group, COPD group and asthma group(P>0.05). (3) The Fe-NO value was significantly lower in smokers of COPD group and ACOS group than that of non-smokers(P<0.05). However, there was no significant difference in FeNO value between smokers and nonsmokers in asthma group and the control group (P>0.05). (4)The smoking index and FeNO value were negatively correlated in COPD group, but there were no obvious cor-relation in smoking index and FeNO values between other groups. Conclusion Smoking can lead to the reduction of FeNO value in COPD and ACOS patients. The detection of FeNO is helpful for the differentiating COPD combined asthma.