The mucociliary clearance apparatus, an important defense mechanism for clearing the lung of bacteria and foreign particulate matter, is a well-coordinated system consisting of airway secretory cells that produce a sol and gel (or mucus) fluid layer on the airway surface and ciliated cells that propel the mucus out of the lung towards the mouth. In vivo mucociliary clearance rates can be measured by following the rate of egress of deposited, radiolabeled markers by gamma camera. Short-acting beta-adrenergic agonists have been shown to enhance mucociliary clearance rates to varying degrees in patients with various lung diseases (eg, asthma, chronic bronchitis, and cystic fibrosis), although the enhancement is generally less than that seen in the normal lung. Limited data on the in vivo dose-response relationships of these mucociliary clearance effects suggest that larger doses are required for enhancement of mucociliary clearance than are needed for bronchodilatation. Little is known about chronic effects, but studies with dosing for up to a week also suggest an enhancement of mucociliary clearance, primarily by agonists that are lipophilic. Issues for future research include the effects of the newer long-acting beta-agonists, large versus small airway effects, and combination effects with other inhaled therapeutic agents (eg, steroids and ion-channel blockers).