This review addresses the use of the metered dose inhaler (MDI) to administer aerosol therapy in the treatment of asthma, bronchitis, and emphysema. Studies have shown that physicians' prescribing patterns for use of the inhaler have been inconsistent with optimal therapy. Furthermore, the medical literature suggests that the metered dose inhaler should replace the jet nebulizer in hospital and outpatient settings as a more efficient and cost-effective treatment method. All classes of aerosol drugs are now available for administration by the MDI. Reports suggest that patients whose conditions do not respond to treatment administered by the MDI may improve following instruction in the proper method of using the inhaler or by increasing the recommended dosage of medication for those receiving beta-adrenergic, anticholinergic, and glucocorticoid drugs. A consensus now recommends that aerosol glucocorticoids be considered the primary method of therapy for asthma; however, the effectiveness of glucocorticoids in the treatment of bronchitis and emphysema has not been determined. Although available data do not prove that drugs used in the treatment of asthma increase mortality, further study is recommended in view of the potential toxicity of these drugs.