Exercise-induced asthma is a common phenomenon, the mechanism of which is undetermined. Eosinophils have been suggested as playing a role in its occurrence. We studied the effect of exercise-induced asthma on the cellular and mediator composition of spontaneously obtained sputum. Twenty-five patients with bronchial asthma were investigated by studying sputum spontaneously obtained before and following challenge. One group with (n=9) and one without (n=9) exercise-induced asthma performed exercise challenge. A third group (n=7) performed methacholine challenge. The sputum was analysed using Giemsa staining for differential cell count, measuring eosinophil cationic proteins and mixtures of leukotrienes (D4, E4 and C4) in the liquid phase using ELISA. The group with exercise-induced asthma had a mean drop of 23.7+/-7.4% in FEV1, significantly (P=0.001) higher than the group without it. Following challenges, there were significant increases in sputum eosinophils only in the group with exercise-induced asthma (from 8.1+/-13.9% to 18.3+/-20.2%, P=0.0017) and not in control groups (from 0.9+/-0.9% to 1.5+/-15%) or in those who had methacholine challenge (from 23.6+/-27.2% to 22.3+/-23.8%). Eosinophil cationic proteins did not change significantly in any group. In the liquid phase of the sputum, the amount of leukotrienes increased following exercise in six of the seven patients with exercise-induced asthma in whom it was measured. The influx of eosinophils to the airway in patients who develop exercise-induced asthma can be partially explained by the leukotrienes in the airways of those patients.