Background and objective: It is not clear whether combination therapy with tiotropium plus formoterol has greater efficacy, without increasing the burden of adverse events, compared with tiotropium alone. This meta-analysis was performed to evaluate the differences in efficacy and adverse events associated with combination therapy compared with tiotropium alone, in patients with stable COPD. Methods: MEDLINE, EMBASE, CINAHL and the Cochrane trials database were searched for this analysis. Randomized controlled trials of 2 or more weeks of treatment with tiotropium plus formoterol or arformoterol, compared with tiotropium alone, were reviewed. Studies were pooled to yield odds ratio (OR) or weighted mean differences (WMD), with 95% confidence interval (CI). Results: Eight trials, involving 1868 randomized patients, met the inclusion criteria. Treatment with tiotropium plus formoterol significantly improved the average FEV1 (WMD 105 mL, 95% CI: 69-142), average FVC (WMD 135 mL, 95% CI: 96-174) and trough FEV1 (WMD 53 mL, 95% CI: 30-76), compared with tiotropium alone, although the difference was not statistically significant for trough FVC. The mean change in transitional dyspnoea index (TDI) was markedly greater with tiotropium plus formoterol (WMD 1.50, 95% CI: 1.01-1.99) than with tiotropium alone, and there was a similar difference in the proportion of patients with a clinically significant change in TDI (OR 2.34, 95% CI: 1.58-3.46). There tended to be fewer adverse events and COPD exacerbations with tiotropium plus formoterol, compared with tiotropium alone, but the differences were not statistically significant. Conclusions: Tiotropium plus formoterol significantly improved lung function and symptom scores compared with tiotropium alone. There was a trend towards a reduction in adverse events, although the difference was not statistically significant. Long-term trials are necessary to evaluate the effects of tiotropium plus formoterol and to clarify the role of combination therapy, compared with tiotropium alone.