We investigated the hypothesis that asthmatic patients have an increased cholinergic tone by measuring tracheobronchial cross-sectional areas during transient voluntary apnea. This allowed us to assess bronchomotor tone without the influence of changes in lung recoil or lung volume. Three groups of subjects with potentially different levels of tracheobronchial tone were studied: 14 healthy volunteers (N), 18 stable asthmatic patients (A), and 10 double lung transplant recipients (T). Using the acoustic reflection technique, we measured changes in tracheobronchial cross-sectional areas during short periods (5 to 10 s) of voluntary apnea. In a subset of subjects, studies were repeated before and after the inhalation of the muscarinic antagonist ipratropium. During breath-holding, glottis and extrathoracic trachea remained unchanged but intrathoracic trachéal area decreased by 30 ± 8% (mean ± standard error of the mean) in N, by 27 ± 3% in A, and by 9 ± 4% in the T group. Bronchial areas decreased by 24 ± 8% in N, by 45 ± 3% in A, and by 10 ± 4% in T. These differences among groups were statistically significant at the trachéal and bronchial levels (p < 0.05), and ipratropium significantly inhibited this airway constriction (p < 0.05) only in the asthmatic group. Assuming that changes in cross-sectional airway areas during voluntary apnea reflect airway tone, these results support the view that in humans this tone is mainly vagally controlled and that it is significantly increased in asthmatic compared with nonasthmatic subjects.