Leukotrienes are proinflammatory mediators with special significance in asthma. Released by numerous cell types, particularly after exposure to allergens, leukotrienes cause a potent contraction of bronchial smooth muscle, resulting in reduced airway caliber. Further, they cause plasma to leak from the vessels, resulting in edema, and enhance the secretion of mucus--both events that increase airway obstruction. Thus, leukotrienes have been a target of basic research in asthma. To date, a number of antileukotriene agents have been developed, and three are currently being used in clinical practice in the United States: zafirlukast and montelukast act by antagonizing the leukotriene receptor, and zileuton inhibits leukotriene synthesis. Studies have shown that these agents improve asthma symptoms, pulmonary function, and patient quality of life. Antileukotriene agents have generally been associated with a low incidence of side effects and good tolerability. Currently recommended for mild-to-moderate, persistent asthma, these agents have enabled patients to reduce their use of corticosteroids.