We studied 27 patients with stable chronic obstuctive pulmonary disease (COPD) in a randomised, single-blind, within-patient, placebo-controlled clinical study. Each patient was assigned on six separate days to receive one of the following drug regimens in random order: A. 40 micrograms ipratropium bromide (Atrovent MDI, 20 micrograms/puff) plus 2 puffs placebo; B. 12 micrograms formoterol fumarate (Foradil MDI, 12 micrograms/puff) plus 3 puffs placebo; C. 80 micrograms ipratropium; D. 24 micrograms formoterol plus 2 puffs placebo; E. 12 micrograms formoterol plus 40 micrograms ipratropium plus 1 puff placebo; F. 4 puffs placebo. On each study day, spirometric indices and vital signs were measured at 5, 10, 15 and 60 minutes, and hourly thereafter up to and including 12 hours after study drug administration. Mean peak FEV1 change (primary endpoint) was maximum with the administration of the combination of ipratropium and formoterol (335.2 ml, SE 24.6), and it differed significantly from the observed peak changes following single administration of the two tested doses of ipratropium (p < 0.05 and p < 0.05 respectively). Safety and tolerability were satisfactory throughout the study.