Montelukast (Singulair) is an antiasthmatic agent that has the chemical structure of a quinoline. Montelukast has a high affinity for the CysLT1 receptor and a potency that is not influenced by human serum protein. Montelukast antagonizes contractions of guinea-pig trachea induced by LTD4 in a competitive manner. Intravenous montelukast inhibited bronchoconstriction induced by LTD4 in guinea pigs. Oral montelukast inhibited increased airway resistance induced by antigen in squirrel monkeys. Montelukast also inhibited both inflammatory and immunologic responses induced by either LTD4 or antigen in guinea pigs and rats. Plasma concentrations of montelukast after oral administration of 10 mg in humans were shown to be over the effective level for at least 24 h. These lines of evidence support the effectiveness of a regimen of 10 mg/day for asthmatic symptoms in humans. In a number of clinical experiments, montelukast not only improved asthmatic symptoms and respiratory indices, but also inhibited airway inflammation and exercise-induced bronchoconstriction. These effects persisted during extended treatment. Montelukast produced an additive effect to basic therapy with an inhaled steroid. There were no differences in the incidence and magnitude of adverse effects between montelukast and placebo groups in clinical experiments. Montelukast is expected to serve as a first line of asthmatic therapy because of its consistent efficacy and good safety profile and it is associated with good compliance in patients because of its simple regimen of one 10 mg tablet/day.