Background. Exercise is a common stimulus of bronchoconstriction in subjects with asthma, who also have bronchoconstriction after inhaling the sulfidopeptide leukotriene D4 (LTD4). The purpose of this study was to investigate the importance of LTD4 as a mediator of exercise-induced bronchoconstriction. Methods. In a double-blind, randomized, crossover study, 12 subjects with stable asthma were treated intravenously with MK-571 (160 mg), a selective and potent LTD4-receptor antagonist, or placebo, 20 minutes before each of two challenges involving exercise at a level previously demonstrated to cause a fall of at least 20 percent in the forced expiratory volume in one second (FEV1). The two exercise challenges were separated by one week. The results of the challenges were expressed as both the maximal fall in FEV1 after exercise and the time to recovery from bronchoconstriction. Results. Treatment with MK-571 attenuated exercise-induced bronchoconstriction in all the subjects. The mean (±SEM) maximal percent decrease in FEV1 after exercise was 25.2±3.5 percent in the subjects taking placebo and 9.2±2.5 percent in the subjects taking MK-571 (P<0.001). Conclusions. The mean percent inhibition for the entire group was 69.5 percent. The mean time to recovery after exercise was 33.4±4.0 minutes in the placebo group and 8.4±2.5 minutes in the MK-571 group (P<0.001). This study demonstrates that pretreatment with a potent and selective LTD4 antagonist markedly attenuates exercise-induced bronchoconstriction, and it suggests that LTD4 is a major mediator of this type of bronchoconstriction. (N Engl J Med 1990; 323: 1736–9.).