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 mg twice daily' should read '18 mg once daily'. The publishers apologise for this error.