Background: In recent years it has been suggested that in COPD, lung volumes can be modified more than expiratory flows, with bronchodilators. Aim: To study the acute effects of salbutamol on FEV1 and lung volumes at rest. Subjects and Methods: Forty stable COPD patients were studied using a single dose of salbutamol (200 µg). Forced expiratory volumen in 1 second (FEV1), slow vital capacity (SVC), forced vital capacity (FVC), and inspiratory capacity (IC) were measured at baseline and after salbutamol administration. Results: After salbutamol, 39/40 patients exhibited a clinically significant increase in volumes (SVC, FVC or IC 10% predicted). A significant increase in FEV1 ( 10% predicted) was observed in only 13 patients. Conclusions: Our results demonstrate that changes in lung volumes, and consequently in dynamic lung hyperinflation, take place more frequently than changes in maximal expiratory flows during the spirometric test in patients with COPD. Assessment of spirometric reversibility based only on changes in FEV1 underestimates the effect of bronchodilator drugs in these patients (Rev Méd Chile 2004; 132: 787-93)