In the paper entitled “COPD exacerbations – 4 · Prevention” by S Scott, P Walker and P M A Calverley which appeared in the May issue of Thorax (2006;61:440–7), the dose of tiotropium used in the studies by Casaburi and Brusasco referred to in table 1 on page 444 which currently reads “18 μg twice daily” should read “18 μg once daily”. The publishers apologise for this error.