Asthma is an inflammatory disease of the airways that is best treated by minimising exposure to factors that provoke the inflammation (e.g. allergens) and by administering drugs that reduce the inflammatory response. The cornerstone of asthma treatment is inhaled corticosteroids. Their effectiveness is a result of their potent and broad anti-inflammatory properties. Antileukotriene drugs (leukotriene modifiers) provide an alternative and novel approach to the treatment of asthma. The novelty of these new compounds is that their effectiveness is firmly based on the pathophysiology of asthma, specifically the role played by the cysteinyl leukotrienes. At the same time, the availability of the antileukotriene drugs has stirred debate over when they should be used and how they compare to inhaled corticosteroids. Although the answers are not fully known at this time, the currently available published and presented data are adequate for us to draw some conclusions about their relative effectiveness and role in asthma treatment. The antileukotriene drugs are more effective than placebo, but they are not as effective as inhaled corticosteroids in improving lung function [measured as the forced expiratory volume in 1 second (FEV1) or peak expiratory flow rate (PEFR)], reducing β2-agonist use, and decreasing symptom-free days. In contrast, they may have similar beneficial effects on reducing asthma exacerbations and decreasing peripheral blood eosinophil counts. In the absence of knowinga priorithe response of an individual patient to treatment with either therapy, the data favour initiating treatment with an inhaled corticosteroid. However, for patients with mild to moderate disease there are a number of circumstances that support using an antileukotriene drug first. A few examples are aspirin intolerance, predominantly exercise-induced symptoms and problems with using an inhaler or the adverse effects of inhaled corticosteroids such as dysphonia and thrush. For patients with more severe disease, inhaled corticosteroids remain the treatment of choice. Antileukotriene drugs should be considered as add-on therapy, especially in view of their possible complementary effects on reducing airway inflammation.