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Background Evidence has demonstrated that the distal lung,which includes airways of ＜2 mm in diameter and lung parenchyma,constitutes an important component of asthma pathology.Cysteinyl leukotrienes (CysLTs) are potent proinflammatory mediators and bronchoconstrictors involved in the asthmatic process.Guidelines recommend the leukotriene-modifying agents for asthma treatment.We hypothesized that a leukotriene receptor antagonist with an inhaled corticosteroid (ICS) and long-acting β2 agonist (LABA) combination would improve small airways function in moderate-tosevere asthmatics evaluated by physiological tests and high-resolution computed tomography (HRCT) analysis.This study was performed at a tertiary university hospital in Beijing.Methods This was a randomized,double-blind,parallel study performed in 38 patients with moderate-to-severe asthma treated with salmeterol/fluticasone (SFC) plus montelukast (SFC+M) or SFC plus placebo over 24 weeks.Small airway function was assessed by physiological studies and HRCT image analysis.Results Montelukast significantly improved air trapping as expressed by the residual volume (RV)/total lung capacity (TLC).Over 24 weeks of treatment,RV/TLC was improved by (15.41±6.67)％ in patients receiving SFC+M while RV/TLC was decreased by (8.57±10.26)％ in patients receiving SFC alone,the difference between the two groups was significant (P=0.02).There was a trend towards a significant difference in forced expiratory volume in the first second (FEV1)/forced vital capacity (FVC) in the SFC+M group compared to that in the SFC group ((17.87±8.17)％ vs.(12.28±9.20)％,P=0.056).There was no significant change in percentage wall area (WA％) after 24 weeks of add-on treatment with montelukast.Patients receiving SFC+M showed significant improvement in the ratio of CT-determined values at full expiration to those at full inspiration (E/I ratio) (0.894±0.005 vs.0.871±0.003,P=0.002).Conclusion We have shown,using lung function tests and HRCT image technique,that add-on therapy with montelukast improves distal lung function reflected by air trapping,but not airway wall thickness in moderate-to-severe asthma.(ClinicalTrials.gov number,NCT00699062)