Of 34 symptomatic adult asthmatic patients (23 men) aged 51 +/- 13 years (mean +/- 1 SD) with moderately severe airways obstruction who underwent maximal exercise testing at room temperature (22 degrees C) and humidity (44 percent RH) using a bicycle ergometer, we identified seven male patients aged 56 +/- 9 years in whom forced expired volume in one second (FEV1) increased greater than or equal to 20 percent over the baseline pre-exercise value (exercise-induced bronchodilation). At maximal exercise, these patients achieved an O2 consumption of 1.4 +/- 0.4 L/min and a minute ventilation of 56 +/- 9 L/min. Baseline FEV1 was 1.3 +/- 0.5 L (SD) (43 +/- 12 percent predicted) and increased to 2.1 +/- 0.5 L at five minutes after exercise and persisted at least 20 minutes. Exercise was repeated in all seven patients on a separate day one to six months later, and results were similar in six. In these seven patients, three minutes of voluntary isocapnic hyperventilation achieving a minute ventilation comparable to that during maximal exercise led to an increase in FEV1 of 20 +/- 18 percent (range 0 to 54 percent). The Vmax50 was 22 +/- 30 percent before, and 10 +/- 21 percent after maximal exercise and 25 +/- 37 percent before, and 11 +/- 22 percent after isocapnic hyperventilation. Pre-treatment with acetylsalicylic acid (mean serum concentration 120 +/- 64 micrograms/ml) in the six patients with reproducible bronchodilation completely blocked exercise bronchodilation in one patient and blunted it in four others. Findings suggest that a subset of adult patients with symptomatic asthma may develop bronchodilation after six to eight minutes of exercise, that exercise-induced bronchodilation may in part be reproduced with isocapnic hyperventilation, and that it may be blocked completely or partially by acetylsalicylic acid, implying mediation by prostaglandins.