The movement of the chest during breathing has been recorded in normal and asthmatic subjects by means of electronic transducers which measured changes in anteroposterior and lateral diameters. Preliminary studies showed that exercise-induced bronchoconstriction caused distortion of the movement of the diameters in relation to each other and to volume change recorded at the mouth. Because of the thoracic gas compression which occurs during breathing, volume change at the mouth does not represent the change in chest volume and further studies were therefore undertaken in a whole-body plethysmograph. These studies showed that the addition of a resistance to the airway of a normal subject distorted the movement of one diameter (usually the anteroposterior) in relation to the other and to volume. With exercise-induced asthma, the movement of both diameters was affected with the anteroposterior lagging behind the volume change. There was an approximate linear relationship between the degree of distortion and airways resistance in the asthmatic subjects and in one normal subject. The physical sign of costal margin paradox could be related to the severity of the obstruction.