Maximal expiratory flow-volume maneuvers were performed by self-trained (FIT) and sedentary (UNFIT) asthmatic subjects. Both groups had similar pre-exercise pulmonary function limitations and attained the same exercising heart rate. The FIT group, however, exercised significantly longer than the UNFIT group. Although expiratory airflow increased in both groups during exercise, the FIT group had significantly larger airflow increases than the UNFIT group and maintained them throughout the exercise. In contrast, the UNFIT group's airflow decreased prior to the end of exercise. Tidal volume (VT) expiratory curves surpassed pre-exercise maximum expiratory flow-volume (MEFV) envelopes in subjects whose tidal volume was greater than 55% of vital capacity, the majority of whom were FIT subjects. In no case, however, did the VT curve exceed the enhanced exercise MEFV curve. The increase in airflow reserve during exercise helps to explain why asthmatic athletes, despite their significantly impaired pulmonary function, can compete successfully in sports making high aerobic demands.