Tracheal mucous velocity was measured by observing the motion of teflon discs across the tracheal mucosa through a fiberoptic bronchoscope. The average rate of movement in 14 adult patients with cystic fibrosis was 2.6 mm per min plus or minus 3.3 SD, compared with 20.1 mm per min plus or minus 6.3 in 20 normal subjects of the same age (P less than 0.001). This failure of mucociliary transport may play a role in the pathogenesis of the pulmonary disease in cystic fibrosis. Administration of a beta-adrenergic agent, terbutaline, increased the average mucous velocity in the patients with cystic fibrosis (to 5.5 mm per min plus or minus 3.6 SD, P less than 0.001) but not in control subjects. This observation has potential therapeutic significance.