Pre-dose spirometry will be conducted at each clinic visit. Six hour post-dose serial spirometry will be conducted at Visit 2 and at Visits 5 and 7. All subjects will be provided with a paper diary for completion everyday throughout the run-in period and 12-week treatment period. Subjects will use the diary to record their daily use of supplemental albuterol/salbutamol and to record any medical problems experienced and any medications used.