Blinded, randomized, and placebo-controlled clinical trials have established that cysteinyl leukotriene (cysLT) receptor antagonists and 5-lipoxygenase (5-LO) inhibitors are safe and effective asthma treatments. Trials of 13- to 26-wks' duration demonstrate that both the cysLT1 receptor antagonist, zafirlukast, and the 5-LO inhibitor, zileuton, improve pulmonary function and decrease daytime and nocturnal symptoms. Concomitant rescue beta-agonist inhaler use and the need for corticosteroid rescue are also reduced. Preliminary studies suggest that antileukotriene agents may also reduce indices of airway inflammation, including inflammatory cell counts and airway hyperresponsiveness. Both cysLT1 antagonists and 5-LO inhibitors offer a new approach to asthma management. Drazen J. Clinical pharmacology of leukotriene receptor antagonists and 5-lipoxygenase inhibitors.