BACKGROUND Although salmeterol and formoterol are both long-acting beta(2) adrenergic receptor agonist bronchodilators, there are distinct differences between them that could translate into differences in clinical response in some patients. OBJECTIVE The goal of this study was to examine the efficacy of formoterol in patients with moderate to severe persistent asthma that was suboptimally controlled with an inhaled corticosteroid (ICS) combined with on-demand salbutamol (albuterol in the United States) with or without salmeterol. METHODS This multicenter, 4-week, randomized, open-label, parallel-group study included adult patients (age >/=18 years) with suboptimally controlled asthma (mean salbutamol use, >/=2 puffs/d via pressurized metered-dose inhaler [100 microg/puff]). Patients were randomized in a 2:1 ratio to receive formoterol 12 microg BID via single-dose dry powder inhaler plus on-demand salbutamol or to continue their existing treatment with either on-demand salbutamol alone or salmeterol 50 microg BID via multidose dry powder inhaler plus on-demand salbutamol. ICS regimens were unchanged during the trial. The primary efficacy variable was evening predose peak expiratory flow (PEF). Secondary variables included further measures of asthma symptom control. RESULTS A total of 6239 adult patients entered the study; data from 6155 patients were available for analysis. Patients who were switched from salmeterol to formoterol reported a significant increase in mean (SD) evening predose PEF compared with patients who continued their existing treatment (402.9 [112.1] vs 385.5 [107.5] Umin, respectively; P < 0.001). Similarly, patients who were switched from on-demand salbutamol alone to formoterol plus on-demand salbutamol reported a significant increase in mean evening predose PEF compared with those who continued treatment with on-demand salbutamol alone (409.3 [105.6] vs 385.0 [105.3] L/min, respectively; P < 0.001). The results for the secondary efficacy measures mirrored the significant improvements seen in patients switched to formoterol compared with those who continued to receive on-demand salbutamol alone or salmeterol plus on-demand salbutamol. CONCLUSION In this study, formoterol significantly improved lung function and control of asthma symptoms and decreased use of rescue medication in patients whose asthma had been suboptimally controlled with an ICS in combination with on-demand salbutamol with or without salmeterol.