Exercise tolerance is an important outcome measure in patients with COPD, mostly because there is evidence that exercise testing is superior to other functional measurements obtained at rest in demonstrating the positive effect of a specific intervention. We assessed the effect of a 5-day treatment with formoterol 12 microg twice daily on lung function, exercise capacity and dyspnea in 22 stable COPD patients, and compared 6-MWT with 12-MWT in evaluating formoterol efficacy. All subjects entered a crossover design. They underwent 6-MWT or 12-MWT in a randomised order and soon after started the 5-day treatment. After a 3-day washout, patients who had first performed 6-MWT, underwent 12-MWT, and the contrary. Formoterol induced a progressively significant increase in pre-drug FEV1 and IC and also significant changes in these parameters 2 h after its inhalation at each test day. Moreover, it increased the walked distance by 53.6 m at the end of 6-MWT and 59.9 m at the end of 12-MWT. Formoterol also induced a significant change in Borg score for dyspnea caused by the 6-MWT when compared with the pre-treatment values, whereas it significantly changed dyspnea induced by 12-MWT only after the first dose. Our study not only strengthens the importance of walking tests as a useful tool for evaluating the impact of a bronchodilator on some COPD patient-centred outcomes, but also indicates that 6-MWT seems to be a more appropriate instrument than 12-MWT for assessing the exercise response to a bronchodilator in COPD.