OBJECTIVE To explore the relationship between leukotriene (LT) and exercise-induced asthma. METHODS Twenty-two non-smoking asthmatic patients with a reproducible fall in FEV(1) after exercise by least 10% and 10 normal patients were enrolled. Urinary levels of leukotriene E4 were measured before and 2 hours after exercise. Twelve subjects received cysteinyl LT receptor antagonist Zafirlukast (Ancolate) 20 mg twice a day for three days. Standardized exercise challenge were performed after 72 hours. End points included (1) area under the percent fall in FEV(1) versus time curve (AUC 0-60 min), (2) time to recovery to within 5% of the pre-exercise baseline FEV(1) value, (3) and maximum percent fall in FEV(1) from pre-exercise baseline. RESULTS To exercise-induced aothma (EIA) group, urinary LTE4 in 2 hours after exercise were higher than that of in before exercise, P < 0.01. Urinary leukotriene E4 concentration before and 2 hours after exercise were higher than normal subjects. Zafirlukast can caused a significant reduction in AUC(0 - 60 min) (Aera under the FEV(1)-time curve for 0 to 60 minutes after exercise); caused a significant reduction in the maximum percent fall in FEV(1); and Zafirlukast resulted in a shorter time to recovery of FEV(1) to preexercise baseline. CONCLUSIONS There was an increase in urinary LTE4 during the 2 hours after exercise challenge; Zafirlukast can attenuate EIA. The results indicate that there is an important role of leukotriene in the pathogenesis of exercise-induced asthma.