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Background The mucolytic and antioxidant effects of N-acetylcysteine (NAC) may have great value in COPD treatment. However, beneficial effects have not been confirmed in clinical studies, possibly due to insufficient NAC doses and/or inadequate outcome parameters used. The objective of this study was to investigate high-dose NAC plus usual therapy in Chinese patients with stable COPD. Methods The 1-year HIACE (The Effect of High Dose N-acetylcysteine on Air Trapping and Airway Resistance of Chronic Obstructive Pulmonary Disease—a Double-blinded, Randomized, Placebo-controlled Trial) double-blind trial conducted in Kwong Wah Hospital, Hong Kong, randomized eligible patients aged 50 to 80 years with stable COPD to NAC 600 mg bid or placebo after 4-week run-in. Lung function parameters, symptoms, modified Medical Research Council (mMRC) dyspnea and St. George's Respiratory Questionnaire (SGRQ) scores, 6-min walking distance (6MWD), and exacerbation and admission rates were measured at baseline and every 16 weeks for 1 year. Results Of 133 patients screened, 120 were eligible (93.2% men; mean age, 70.8 ± 0.74 years; %FEV 1 53.9 ± 2.0%). Baseline characteristics were similar in the two groups. At 1 year, there was a significant improvement in forced expiratory flow 25% to 75% ( P = .037) and forced oscillation technique, a significant reduction in exacerbation frequency (0.96 times/y vs 1.71 times/y, P = .019), and a tendency toward reduction in admission rate (0.5 times/y vs 0.8 times/y, P = .196) with NAC vs placebo. There were no significant between-group differences in mMRC dypsnea score, SGRQ score, and 6MWD. No major adverse effects were reported. Conclusion In this study, 1-year treatment with high-dose NAC resulted in significantly improved small airways function and decreased exacerbation frequency in patients with stable COPD. Trial registry ClinicalTrials.gov ; No.: NCT01136239 ; URL: www.clinicaltrials.gov