Leukotriene receptor antagonists (LTRAs), such as montelukast and zafirlukast, have been demonstrated in a number of studies to possess bronchodilating and anti-inflammatory properties, that make these drugs ideal candidates for the treatment of asthma. The last 1998-updating of the GINA Guidelines for the diagnosis and therapy of asthma recommends the use of LTRAs in the treatment of moderate and mild asthma. In patients with moderate asthma not completely controlled with moderate doses of inhaled corticosteroids, the addition of a LTRA is indicated in alternative to either the increase of the inhaled corticosteroid dose or the addition of an inhaled long-acting beta2-agonist. Both in vitro and in vivo evidences indicate that LTRAs possess an anti-inflammatory activity that is presumably complementary to that presented by corticosteroids. Moreover, clinical studies show that the addition of an LTRA, montelukast, is able to improve clinical and functional indexes in patients with asthma not controlled with inhaled corticosteroids, and to allow a reduction in corticosteroid dosage in patients with asthma well controlled by inhaled corticosteroids. In patients with mild persistent asthma monotherapy with an LTRA is indicated in alternative to a low-dose inhaled corticosteroid, an inhaled cromone, or an oral slow-release theophylline. Previous clinical studies in patients with mild to moderate asthma had demonstrated that monotherapy with LTRAs is able to improve airway function, asthma symptoms, use of as-needed medications, exacerbation rate, and quality of life, without evidence of tolerance with prolonged use. Recently, in a subgroup analysis of patients with mild persistent asthma, a 6-week treatment with oral montelukast or inhaled beclomethasone gave similar improvements in "rescue-free" days, days with well controlled asthma, FEV1, blood eosinophils, beta-agonist use, and nocturnal awakes due to asthma.