Exercise-induced asthma was studied in 8 asthmatics using various conditions of inspired air during exercise. The exercise consisted of walking on a treadmill for 10 min, with a speed and grade elevation adjusted to achieve the target heart rate of approximately 90% of predicted maximum. Pulmonary function tests were performed pre- and post-exercise to determine exercise-induced asthma. With inspired air at 23 degrees C and 15% relative humidity (RH), the post-exercise forced expiratory volume in a sec (FEV1), maximal mid-expiratory flow rate (MMEF), and specific airway conductance (SGaw) decreased to an average of 69, 59 and 38% of the pre-exercise baseline, respectively. In contrast, the exercise-induced asthma was clearly prevented in all subjects by using inspired air at 37 degrees C and 100% RH, when the post-exercise FEV1, MMEF, and SGaw were 99, 100 and 91% of the baseline, respectively. Inspiration of warm, dry air or humid, room air reduced but did not prevent exercise-induced asthma. The results indicate that the primary stimulus for exercise-induced asthma may be heat loss and/or water loss from the airways during exercise.