Unlike conventional methods, the interrupter method for measuring airway resistance is non-invasive and requires minimal patient co-operation. It can therefore be applied in critically ill patients, acute asthmatics, neonates, pre-school children, geriatric patients and unconscious patients. The method is based on transient interruption of airflow at the mouth for a brief period during which alveolar pressure equilibrates with mouth pressure. Measurement of mouth pressure is used to estimate alveolar pressure prior to interruption and the ratio of this to flow prior to interruption gives airway resistance. Using the interrupter method we have developed a portable device for measuring airway resistance which is simple to use and gives a direct instantaneous reading. Measurements of airway resistance obtained using the new device were compared with those obtained using conventional body plethysmograph methods in 43 adult patients. A close correlation was seen (r = 0.86). The two methods appear equally sensitive in detecting changes in airway resistance following bronchodilator therapy. The device has been used successfully in pre-school children unable to co-operate with conventional methods.