The aim of our study was to evaluate the pharmacodynamic effects of 1-day treatment with formoterol, tiotropium and their combination in patients with acute exacerbation of chronic obstructive pulmonary disease (AECOPD). Twenty-one (19 males, mean age 72+/-8 years, mean FEV1 38+/-14% of predicted values) patients with mild to moderate AECOPD were enrolled. Patients received formoterol (12 microg deliver via Modulite b.i.d.), tiotropium (18 microg dry powder capsules delivered via HandiHaler once daily), and their combination, in randomized sequence. Serial measurements of FEV1, FVC, IC, SpO2 and HR were performed over 24h. Formoterol, tiotropium, and their combination significantly improved the area under curves (AUCs) for FEV1, FVC and IC over 12 and 24h. The mean FEV1, FVC and IC AUC(0-12h) and AUC(0-24h) after formoterol and tiotropium combination were significantly higher than formoterol and tiotropium alone, whereas the differences between the two single drugs were not statistically significant. Formoterol, either alone or in combination with tiotropium, elicited a significantly faster onset of action, and combination elicited a greater maximum bronchodilation than both single drugs in terms of FEV1 and FVC. After 24h the bronchodilating effect of the three treatments disappeared, with the exception of the combination on FEV1. The results of this study have documented that, although the time course of the effects of evaluated drugs differs significantly from that in stable COPD, with a shorter bronchodilation both for tiotropium and formoterol, these two long-acting bronchodilators appear to also be complementary in mild to moderate AECOPD.