A new add-on spacer design concept for a commercial dry powder inhaler (Turbuhaler(R), Astra Pharma Inc., Mississauga, Ontario) is proposed here. Design partial optimization is performed using computational fluid dynamics. The performance of a simplified straight diffuser and a partially optimized geometry are evaluated by measuring the impact of these add-on devices on efficiencies of aerosol deposition in an idealized mouth-throat cast. HPLC and UV spectroscopy techniques are used in the aerosol deposition measurements. Turbuhaler(R) (terbutaline sulfate, 500 mug) aerosol and an inhalation flow rate of 70 l/min are used for both simplified and partially optimized add-on devices. In addition, monodisperse aerosols for three inertial parameter values, rho(p)d(p)(2)Q(g mum(2) s(-1)), of 6635, 14,700 and 23,430 are used for the partially optimized geometry. Compared to Turbuhaler(R) alone, the newly developed and partially optimized spacer significantly improved total aerosol delivery distal to the mouth-throat cast by approximately 47% for Turbuhaler(R) aerosol and 17%, 27% and 107%, respectively, for the above monodisperse aerosols. In contrast, the straight diffuser did not significantly improve delivery distal to the mouth-throat replica. (C) 2004 Elsevier Ltd. All rights reserved.