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Several clinical trials have shown that the inhaled β 2 -agonists with long-acting properties, formoterol and salmeterol, may be effective in acute exacerbations of chronic obstructive pulmonary disease (COPD). However, there is a great deal of controversy regarding the timing and optimal dose of inhaled β 2 -agonists in this pathologic condition. In this double-blind, randomised, crossover study, we have compared the bronchodilating effect and the safety of inhaled formoterol administered via Turbuhaler using either a cumulative dose regimen or the equivalent single dose in 16 patients with acute exacerbations of COPD. On the two consecutive days, the patients received, in a randomised order, each of the following active dose regimens: (A): 9 + 9 + 18 μg of formoterol via Turbuhaler (36 μg cumulative delivered dose), or (B): 36 + 0 + 0 μg of formoterol via Turbuhaler. The three doses on each treatment day were administered at 30-mm intervals, with measurements being made 5 and 30 min after each dose. Contemporaneously, we also measured oxygen saturation by pulse oximetry ( S pO 2 ) and pulse rate. Both the high dose and the cumulative one induced a significant bronchodilation expressed as change in FEV 1 . The difference between the two regimens was significant ( P =0.0332) only 60 min after the first inhalation. The trend of FVC and IC was similar to that of FEV 1 . All treatment regimens were well tolerated and no adverse events were reported. Neither the administration of the high dose nor that of the cumulative one modified heart rate in a significant manner. Also they did not influence S pO 2 . This study indicates that a single high dose of formoterol is as effective as the same dose administered in a cumulative manner in patients with acute exacerbation of COPD.