The purpose of this study is to test if Symbicort® (budesonide/formoterol) Turbuhaler® is effective in treating asthma when used 'as needed' in patients with milder asthma. The efficacy of Symbicort® 'as needed' will be compared with: 1) terbutaline Turbuhaler® 'as needed' and with 2) Pulmicort (budesonide) Turbuhaler® twice daily plus terbutaline Turbuhaler® 'as needed'.