We conducted a randomized, crossover trial with tiotropium 18 microg once daily (group A), and formoterol 12 microg twice daily (group B) over a 5-day period for each drug, with a 10-day washout, in 20 COPD patients. At the end of each period, patients inhaled both drugs separated by 180 min in alternate sequence (group A: tiotropium 18 microg+formoterol 12 microg; group B: formoterol 12 microg+tiotropium 18 microg). FEV1 and FVC were measured at baseline and after 30, 60, 120, 180, 210, 240, 300 and 360 min. FEV1 and FVC further improved after crossover with both sequences. The mean maximal change in FEV1 over baseline was 0.226 L (0.154-0.298) after tiotropium+formoterol and 0.228 L (0.165-0.291) after formoterol+tiotropium; the mean maximal change in FEV1 over pre-inhalation the second drug value was 0.081 L (0.029-0.133) after tiotropium+formoterol and 0.054 L (0.016-0.092) after formoterol+tiotropium. The mean maximal change in FVC over baseline was 0.519 L (0.361-0.676) after tiotropium+formoterol and 0.495 L (0.307-0.683) after formoterol+tiotropium; the mean maximal change in FVC over pre-inhalation of the second drug value was 0.159 L (0.048-0.270) after tiotropium+formoterol and 0.175 L (0.083-0.266) after formoterol+tiotropium. The FEV1 AUCs(0-360 min) were 62.70 (45.67-79.74) after tiotropium+formoterol and 69.20 (50.84-87.57) after formoterol+tiotropium, the FEV1 AUCs(0-180 min) were 24.70 (18.19-31.21) after tiotropium+formoterol and 29.74 (21.02-38.46) after formoterol+tiotropium, whereas the FEV1 AUCs(180-360 min) were 15.70 (10.88-20.52) after tiotropium+formoterol and 11.71 (7.21-16.21) after formoterol+tiotropium. Differences between the two treatments were not statistically significant (P>0.05). The addition of second different long-acting bronchodilator to a regularly administered long-acting bronchodilator seems to be to patient's advantage.