Background: The optimal treatment for patients with mild asthma is uncertain. This study evaluated the effects of adding inhaled formoterol, a fast onset and long-acting beta agonist, to low dose inhaled budesonide for 1 year, in patients with mild persisting asthma who were taking a low dose of inhaled corticosteroids (≤400 ug/day). Methods: The 1272 corticosteroid treated patients (mean baseline FEV1 86.54%) were assigned to twice daily treatment with 100 ug budesonide, or 100 ug budesonide plus 4.5 ug formoterol, or 200 ug budesonide, or 200 ug budesonide plus 4.5 ug formoterol. The main outcome variables were the time to the first severe asthma exacerbation and poorly controlled asthma days. Results: The addition of formoterol either to a lower or higher dose of budesonide reduced the risk for the first asthma exacerbation (RR = 0.57, 95% CI = 0.46, 0.72) and the rate of poorly controlled asthma days (RR = 0.70, 95% CI = 0.60, 0.82). Adding formoterol also increased FEV1 and morning PEF, reduced asthma symptoms, the number of rescue inhalations of short-acting inhaled beta-agonists and nocturnal awakenings, although the latter did not reach statistical significance Comparison of the groups taking budesonide 100 ug bd and 200 ug bd showed only boderline statistically significance in the risk for first severe asthma exacerbation, or rate of poorly controlled asthma days. Asthma symptoms, FEV1 and morning PEF, however, improved more with the higher dose of budesonide. Adding formoterol to the lower dose of budesonide reduced the risks for a severe asthma exacerbation or a poorly controlled asthma day and improved lung function more than giving the higher dose of budesonide alone. Conclusions: In mild asthma patients, already taking a low dose of inhaled corticosteroids, risks for severe asthma exacerbations and poorly controlled asthma days were reduced by adding inhaled formoterol.