Background: The six minute walk test (6MW) elicits dynamic hyperinflation (DH) in severe COPD patients, which can be evaluated by reductions in inspiratory capacity (IC). Although IC is currently used to determine the effects of bronchodilators on DH during exercise tests on a cycle ergometer, its usefulness during a walking test has not been evaluated. Aim: To study the acute effects of ipratropium bromide (IB) on forced expiratory volume at l second (FEV1) and IC at rest and on DH during exercise assessed by the 6MW. Subjects and methods: Fifteen stable COPD patients were randomly allocated in a double-blind, placebo-controlled, crossover fashion to 2 treatment periods using a single dose of nebulized IB 500 mg or placebo. Spirometry, including IC, and 6MW were measured at baseline and after IB and placebo. IC was also measured 15 min after exercise. Dyspnea, oxygen saturation (SpO2) and heart rate were assessed at the end of exercise. Results: After IB, 8/15 patients exhibited a clinically significant increase in IC (<FONT FACE=Symbol>³1</FONT>0% predicted). A similar increase in FEV1 was observed in only one patient. No changes were observed with placebo. A significant increase in 6MW from baseline (p=0.007) was found after IB (45±14 m) compared to placebo (0.5±9 m), whereas dyspnea was significantly lower. Inspiratory capacity fell after 6MW with both treatments, but it reached their baseline values at 15 min after exercise only with IB. Conclusions: Our results demonstrate that IC provides additional information to conventional spirometry on the acute effects of bronchodilators and confirm its value to assess DH during a walking test (Rev Méd Chile 2003; 131: 605-612)