The degree of post-exercise airway obstruction (Exercise-Induced Asthma (EIA] in 14 children was compared to the degree of airway obstruction following isocapnic hyperventilation. EIA was provoked by 6 min of treadmill running. Isocapnic hyperventilation was performed sitting during 6 min. The total ventilation (Vtot) during the two provocations was identical. The temperature of the inspired air was also identical during the two provocations, and the relative humidity was 40% during treadmill-running and 15% during hyperventilation. The decrease in peak expiratory flow after treadmill-running was 29%. After hyperventilation a fall on 19% was seen. These figures are statistically different. It is concluded that although there is a significant difference in airway obstruction after the two provocations the ventilation is greater importance for EIA than is the work load.