This is a Phase IIb, multi-centre, randomised, double-blind, parallel-group, placebo-controlled study in children aged 5-11 years with persistent uncontrolled asthma. Subjects entering the run-in period will stop their current asthma medication and be given open label fluticasone propionate (FP) 100mcg twice daily via DISKUS/ACCUHALER and salbutamol/albuterol as required to use throughout the run-in and double-blind treatment period. At Visit 3 subjects meeting the randomization eligibility criteria will receive vilanterol (6.25mcg, 12.5mcg, or 25mcg,) or placebo via the Novel Dry Powder Inhaler (NDPI) once daily for 4 weeks in addition to open-label fluticasone propionate twice daily throughout the treatment period. Primary endpoints consist of change from baseline in clinic visit trough (pre-bronchodilator and pre-dose) PEF at the end of the 28-day treatment period in all subjects. Safety assessments include adverse events, oropharyngeal examinations, clinical chemistry, 12-lead ECG, and vital signs. Blood samples will be taken from all subjects for pharmacokinetic analysis to determine plasma concentrations of vilanterol at specific time intervals relative to the dose of study drug.