Leukotriene antagonists are effective in the treatment of exercise-induced bronchoconstriction. Montelukast is a specific cysteinyl-leukotriene receptor1 antagonist without known effects on the pulmonary vessels, which in theory should be advantageous with respect to gas exchange. In addition to lung function, we investigated the effects of montelukast on parameters of gas exchange and physical performance in 16 asthmatics with exercise-induced bronchoconstriction in a double-blind cross-over placebo-controlled study. Subjects were tested at an ambient temperature of -15 degrees C with a tread mill exercise protocol consisting of consecutive workloads of 80% V'O(2max) (6 min), rest (4 min), 60% V'O(2max) (6 min) and step increments of exercise until exhaustion. Montelukast reduced the maximum post-exercise fall in FEV1 (P < 0.01), improved the running time to exhaustion in 11 of 16 test subjects (one unchanged) (P = 0.03), and reduced the Borg score at exhaustion (P = 0.03) and the breathing frequency after 3 min at 60% V'O(2max) (P = 0.03). V'(O2), V'CO(2), minute ventilation, ventilatory equivalents, respiratory exchange ratio, heart rate and oxygen pulse were not significantly different after montelukast and placebo. We conclude that montelukast has a beneficial effect on physical performance in most adults with exercise-induced asthma without any observed effect on gas-exchange parameters.