Urinary levels of leukotriene (LT) E4, a stable end-product of LTC4 and LTD4, were measured before and after exercise in 10 children with severe asthma and seven children with moderate asthma using HPLC and RIA to clarify the relationship of LT to the severity of asthma and to the degree of bronchospasm in exercise-induced asthma. The urinary LTE4 level significantly increased after exercise in the severe asthma group, but not in the moderate asthma group (14.3 +/- 14.5 to 24.3 +/- 20.6 versus 19.6 +/- 12.3 to 17.6 +/- 10.8 ng/mmol creatinine, p less than 0.05). The urinary LTE4 level increased in 10 patients (eight with severe asthma), and it decreased in seven patients (five with moderate asthma). A significant difference in the degree of bronchospasm after exercise (as shown by the maximal % fall in the peak expiratory flow rate), was seen when patients with increased urinary LTE4 excretion were compared with those with decreased excretion (60.4 +/- 17.3 versus 24.1 +/- 14.3%, p less than 0.01). Our findings suggest that exercise-induced asthma, or at least a subtype of exercise-induced asthma, may partly develop through the release of LTC4.