The present study was aimed to examine (i) the overall energy input (E-dispersion) into two capsule-type pharmaceutical powder inhalers, the Dinkihaler(R) and the Rotahaler(R), and (ii) the effect due to the presence of capsule, on powder dispersion using mannitol as a model compound. Powder dispersion was assessed by a liquid impinger with the fine particle fraction (FPF) defined as the wt.% of particles less than or equal to5 mum in the aerosol. The overall energy input is given by E-dispersion = DeltaPV(aftdispersion), where DeltaP is the total pressure drop across the inhaler, V-af the volume air flow through the inhaler, and t(dispersion) the time taken for powder to empty out of the capsule. The overall energy input, E-dispersion, was found to correlate with the FPFs generated by either inhaler. Compared with the Rotahaler(R) at the same air flows, the Dinkihaler(R) had larger E-dispersion values, due to higher DeltaP and longer t(dispersion). The larger E-dispersion was consistent with the higher FPFs. For both inhalers, the presence of a capsule improved the FPFs, presumably via collision between the powder and capsule during dispersion. For the Dinkihaler(R) the capsule-end apertures also affected the FPF, powder emptying and impaction loss. The optimal aperture size was found between 1.00 and 2.38 mm to deliver a high FPF (50-60 wt.%) with minimal impaction loss and capsule and device retention at a moderate air flow (0.0015 m(3)/s). (C) 2002 Elsevier Science Ltd. All rights reserved.