Metered-dose inhalers (MDIs) are an effective means of generating drug-containing aerosols targeted for delivery to intrapulmonary airways. Many problems associated with incorrect patient use of MDIs are mitigated by adding a valved spacer device to the inhaler mouthpiece. This in vitro study compared the efficiency of drug output through a new spacer device, OptiChamber (HealthScan Products Inc., Cedar Grove, NJ), to that of a device commercially available since the 1980s, AeroChamber (Monaghan Medical, Plattsburgh, NY). Testing utilized MDI formulations of albuterol, beclomethasone dipropionate, and cromolyn sodium. OptiChamber equaled or, in the majority of cases, exceeded AeroChamber in output of the three drugs at two simulated inspiratory flow rates. Drug output from OptiChamber was found to be less sensitive to changes in flow rate than that from AeroChamber. OptiChamber also showed less decrease in drug output than AeroChamber when time delays were introduced between MDI actuation and the start of a simulated inhalation. Mass median aerodynamic diameters of drugs exiting the two spacers were generally similar to those of drugs exiting the MDI alone. However, spacers were shown to nearly eliminate the output of large-size drug particles (>5.8 microm), which can result in oropharyngeal drug deposition. Emitted fine-particle drug (<5.8 microm) doses from OptiChamber were greater than those from AeroChamber with or without a delay between canister actuation and the start of a simulated inhalation. The results suggest that OptiChamber may provide more efficacious aerosol drug delivery than AeroChamber under both ideal and suboptimal conditions.