Previous studies have suggested that mucociliary clearance (MC) is impaired in asthmatic subjects. If so, impaired clearance may be an important factor in acute exacerbation. We proposed that if MC plays a significant role in acute illness, MC should be impaired during the exacerbation but improve after recovery. To test this hypothesis, five asthmatic patients with attacks requiring hospitalization underwent measurement of MC using radiolabeled aerosol and a gamma camera. They were studied on the second or third day after admission with repeat measurements after discharge. Spirometry was performed before all studies. After an equilibrium xenon scan (133Xe), which defined lung borders and measured regional volume, radiolabeled saline particles containing technetium-labeled (99mTC) sulfur colloid were deposited and used to label airway mucus. Deposition patterns were matched by regulating particle distribution and breathing pattern. MC was then measured as percentage retention of radioactivity at 10-min intervals for 2 h. When hospitalized, 96.0 +/- 2.06% (SEM) of the initial radioactivity was retained in the lung after 2 h, indicating little clearance of mucus from the lung. In fact, no significant changes were detected when activity at 120 min was compared with measurements at 10 min (99.2 +/- 0.22%, NS). After discharge clearance was markedly enhanced. That is, retention of lung activity was significantly lower at all time intervals from 10 min onward, with only 70.9 +/- 3.86% retained at 120 min (p = 0.008). During an asthmatic attack warranting hospital admission, MC is significantly impaired, with marked improvement following recovery.(ABSTRACT TRUNCATED AT 250 WORDS)