Metered-dose inhalers (MDI) are routinely used to administer inhaled antiasthma drugs. Actuation-inhalation coordination problems are overcome and systemic side effects are reduced by using spacers/holding chambers (SP/HCHs). Many of these devices do not allow the use of the manufacturer's actuator. The objectives of this study were (a) to investigate the effect of the interaction of eight MDI products with four different SP/HCHs on their metering performance (MP); and (b) to test the hypothesis whether the MP obtained with a SP/HCH and a given drug (MDI) can be extrapolated to other MDIs, even for members of its particular drug class. The procedure outlined in The United States Pharmacopeia-The National Formulary was used (determination of canister weight changes after actuation). The SP/HCH tested were Aerochamber, Inspirease, and ACE. The MDIs tested were salmeterol xinafoate; albuterol with chlorofluorocarbons and 1,1,1,2-tetrafluoroethane as propellants; cromolyn sodium; nedocromil sodium; flunisolide; beclomethasone dipropionate; and fluticasone propionate. Only flunisolide-Inspirease presented an unacceptable MP. Although within the acceptable limits, the MP varied significantly between the following MDI-SP/HCH combinations: Optihaler-fluticasone propionate and Optihaler-cromolyn sodium < to Aerochamber-fluticasone propionate and Aerochamber-cromolyn sodium (p = 0.0015 and p = 0.0007, respectively); and Inspirease-flunisolide and Optihaler-flunisolide < Aerochamber flunisolide (p = 0.003 and p = 0.005, respectively). MP did not significantly vary when albuterol with chlorofluorocarbons or 1,1,1,2-tetrafluoroethane as propellants, salmeterol xinafoate, beclomethasone dipropionate, and nedocromil sodium were attached to any of the SP/HCHs studied. Our results emphasize the capital importance of choosing the right combination of MDI and SP/HCH for aerosol delivery. The MP obtained with a drug and a SP/HCH cannot be expected to be similar for other MDIs, even for members of its drug class. These data also suggest the need for regulatory agencies to approve an MDI to be used only with the SP/HCHs tested.