Editor,—In his recent letter, Dr Mitchell was concerned that the methodology used in our study did not simulate the release of aerosol from a metered dose inhaler (MDI).1However, as we discussed, the method of aerosol delivery in our study differs from that of a MDI but, because the delivery system was kept constant and the particular spacer varied, a valid comparison of the efficacy of different spacers could be made.2 This delivery system has been previously …