Thirty-six adult patients (16 women) with mild to moderate asthma with a mean baseline forced expiratory volume in 1 sec (FEV1) of 73.8% (46-106%) of predicted normal value and mean reversibility of 24.2% (14.6-47.1%) were included in this double-blind, double-dummy, randomized, placebo-controlled and cross-over study. The patients inhaled single doses 4.5 or 9 microg of formoterol (Oxis) via Turbuhaler salbutamol (Ventolin) 100 or 200 microg from a pressurized metered dose inhaler (pMDI) or placebo at five randomized visits. Efficacy was measured by FEV1 pre-dose and then 1, 3, 5, 7, 10, 15, 20, 25 and 30 min after inhalation of the study drug. The primary variable of efficacy was the FEV1-value 3 min after dose intake. No statistically significant differences were found between active treatments. All active treatments gave a higher bronchodilating effect at 3 min than placebo: 10.0, 11.4% for salbutamol 100 and 200 microg and 11.7, 11.8% for formoterol 4.5 and 9 microg (P<0.001 in all cases). There was a correlation between the measured response at 3 min and the subjective experience of the patients. The relative difference vs. placebo remained throughout the study period for all active treatments except for low dose salbutamol. All treatments were well tolerated. In conclusion, formoterol Turbuhaler has as rapid an onset of action as salbutamol pMDI when given at recommended doses.