The present study compared the safety of 4.5 microg formoterol with 0.5 mg terbutaline, both by Turbuhaler and used as needed, in addition to regular formoterol in moderate asthma. In this double-blind parallel-group study, 357 patients taking a moderate-to-high dose of inhaled corticosteroids and additional terbutaline (2-5 inhalations x day(-1) during run-in) were randomised to either formoterol or terbutaline as needed in addition to formoterol 9 microg b.i.d. over 12 weeks. Adverse events, serum potassium levels, electrocardiogram, vital signs and lung function were assessed monthly; peak expiratory flow and severe asthma exacerbations were recorded daily. Patients used 2.16 (range 0.0-6.3) formoterol and 2.34 (range 0.1-7.5) terbutaline relief inhalations x day(-1). No clinically significant differences in safety variables were found between treatments. Statistically greater increases in cardiac frequency (2.6 beats x min(-1), p=0.03) were found on terbutaline. There were 44 and 52 severe asthma exacerbations with formoterol and terbutaline, respectively, with no significant difference in time to first exacerbation. There was also no difference between treatments for other efficacy measures (peak expiratory flow, forced expiratory volume in one second and morning/evening symptom scores). Formoterol 4.5 microg as needed was at least as safe, well tolerated and effective as terbutaline 0.5 mg in stable patients (requiring up to 6 relief inhalations x day(-1)) taking formoterol plus inhaled corticosteroids regularly over 12 weeks.