The effect of breathing a gas mixture less dense than room air (80% helium, 20% 02) on the maximal expiratory flow-volume curve was used to determine the influence of clinical factors on the site and nature of the airway obstruction in asthmatics. If an asthmatic did not smoke or have chronic bronchitis or recurrent respiratory infections, the site of obstruction was in more central airways than it was in the presence of these factors, when the main site was more likely to be in more peripheral airways. Both the peripheral and the central airway obstruction were at least in part due to bronchoconstriction and could be reversed with bronchodilators. Comparison of maximal expiratory flow-volume curves when the subject was breathing air and when he or she was breathing a less dense gas mixture may indicate change in the mechanical properties of the lung after the use of bronchodilators that are not apparent breathing air alone. In some instances, bronchodilators produced no change in the maximal expiratory flow-volume curve when the subject was breathing air, but there were substantial changes when he or she was breathing a mixture of helium and O2.