N-acetylcysteine (NAC) is an antioxidant and anti-inflammatory. Its effects on chronic obstructive pulmonary (COPD) outcomes, including exacerbation of and changes in lung function parameters, are controversial. To investigate the effects of NAC on COPD exacerbation and changes in lung function parameters in patients with COPD. A meta-analysis of randomized controlled trials retrieved from PubMed and Medline databases (12 trials; 2691 patients). High-dose [relative ratio (RR) = 0.90, 95% confidence interval (CI) = 0.82–0.996, P = 0.041] and low-dose (RR = 0.83, 95% CI = 0.69–0.99, P = 0.043) NAC reduced COPD exacerbation prevalence. Long-term (≥6 months), but not short-term, NAC reduced exacerbation prevalence (RR = 0.85, 95% CI = 0.74–0.98, P = 0.024). NAC did not affect exacerbation rate, forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), or inspiratory capacity (IC). Long-term NAC therapy may reduce risk of COPD exacerbation.