Respiratory acoustical impedance, a new method to measure airway obstruction during quiet breathing, was compared with plethysmography and spirometry in 19 asthmatics undergoing a bronchial inhalation challenge with an allergen. Respiratory acoustical impedance and plethysmography were both more sensitive than spirometry in detecting bronchoconstriction. Respiratory acoustical impedance is easily measured during quiet breathing, even in uncooperative subjects; forced expirations, which may alter bronchial tone, are avoided. Respiratory acoustical impedance is sensitive and reliable in measuring acute airway obstruction.