Glottic caliber is modulated in a way that optimizes the airway resistance and the work of breathing. The midexpiratory glottic narrowing that normally occurs during tidal breathing is enhanced during histamine- or methacholine-induced airway narrowing. Exercise is associated with increased ventilatory demand and midexpiratory glottic widening in healthy subjects. However, the relative contribution of glottic aperture to airflow limitation during exercise-induced asthma is unknown. Consequently, we used the acoustic reflection technique to test whether a decrease in midexpiratory glottic size occurred also during exercise-induced bronchoconstriction in seven asthmatic subjects. Exercise resulted in a 38 +/- 16% (SD) fall in FEV1 from the baseline value (p less than 0.001) and a significant increase in midexpiratory glottic cross-sectional area from 1.89 +/- 0.45 cm2 before to 2.36 +/- 0.71 cm2 during bronchoconstriction (p less than 0.05). This midexpiratory glottic widening was also associated with increased tidal flow rates. We conclude that in marked contrast to airway obstruction induced by histamine or methacholine, exercise-induced asthma is associated with midexpiratory glottic widening, which occurs concomitantly with a decrease in lower airway caliber. We suggest that modulation of glottic aperture during acute asthma is heterogeneous and depends partly on the initiating stimulus.