Spacers are commonly prescribed for use with pressurized metered dose inhalers (MDIs), in the hope of increasing the total drug dose inhaled by a patient. The effect of spacer geometry, size, design and mode of use on the quality of aerosol output was critically examined to assess the validity of typical in-vitro test protocols used by spacer manufacturers to document spacer performance. (1) A pilot clinical study showed that patients induced delays between actuation and the onset of inhalation, even after extensive training on AeroChamber, ACE and OptiHaler spacers. Total drug output (TDO) from these spacers was reduced substantially when such delays were simulated in-vitro. (2) AeroChamber and OptiChamber (differently sized spacers with mouthpiece valves) were tested with Ventolin, Beclovent and Intal to determine TDO and fine particle dose (FPD). OptiChamber yielded a higher TDO and FPD, which was less sensitive to actuation-inhalation delays than AeroChamber. Spacer size and valve design were found to affect performance. (3) Spacers 2 to 8 inches long, with internal diameters of 1 to 2.5 inches were tested with Ventolin and Intal. For Ventolin, the 2.5 inch spacer gave the highest TDO and FPD of all the lengths tested. Length had a minimal effect. Drug output from Intal was affected by both spacer length and width. (4) Sites of maximum drug loss within these spacers were determined by building equivalently sized spacers from interlocking rings. Ventolin deposition was concentrated 1 to 2 inches from the actuator spray nozzle, while almost uniform Intal distribution was observed along the spacer length. (2) and (3) suggest that all spacer-MDI combinations may not function equivalently. Determining spacer performance under ideal in-vitro conditions, and ignoring patient use scenarios can potentially over estimate spacer efficiency. All valved spacers do not perform identically and larger spacers do not always enhance dose delivery. There are significant differences in meaningful performance parameters of marketed spacers deemed equivalent by the United States Food and Drug Administration. Spacer performance with one inhaler may not be indicative of all inhaled medications.