Facemasks are widely used with valved holding chambers (VHCs) that are prescribed for use with pressurized metered dose inhalers for patients who have difficulty coordinating inhalation with inhaler actuation. Adults also require this type of interface, but there is a wide variety of facial profiles that must be accommodated to minimize dead space and the risk of leakage of ambient air at the facemask-to-face seal. We describe several key steps that were taken to design a slightly smaller size of facemask to accompany the current large adult facemask supplied with the AeroChamber Plus* Flow-Vu* VHC. The desired facemask size lies between the sizes of the medium (child) and large (adult) facemask sizes. The new facemask is primarily intended for adults having smaller facial features. We describe the use of anthropomorphic data pertinent to this class of user that, in conjunction with the identification of key dimensions, enabled the development of prototype facemask designs for optimisation via user evaluation. Each prototype was evaluated in terms of of a) inhalation flow rate as a measure of face-to-facemask fit, and b) user preference for comfort / fit. The prototype selected for progression performed best overall in terms of flow rate and fit. The aim of this abstract was to describe the process followed in order to design and optimize, with respect to fit and comfort, a new VHC facemask developed for adults with smaller facial features.