The phase out of chlorofluorocarbon (CFC) propellants has necessitated the reformulation of pressurized metered dose inhalers (pMDIs) with hydrofluoroalkane (HFA) propellants. One of the main challenges has been that conventional surfactants used for CFC-based pMDIs were not soluble in HFAs. Since one of the main aims of a pMDI is to deliver a reproducible dose of medication to the patient, it is vital that, for suspension-type pMDI formulations, the suspension is stabilized sufficiently for a reproducible dose to be delivered. A new technique has been developed that measures suspension stability more objectively than before. This technique, optical suspension characterization, was used to compare the performance of different formulations of respiratory drugs in HFAs. The optical suspension characterization data were correlated with conventional analytic techniques for the determination of the stability of the suspension formulation. (J Allergy Clin Immunol 1999;104:S221-6.)