 Multiple genetic polymorphisms in the leukotriene pathway have been described but their clinical relevance is unclear. A single nucleotide polymorphism in the LTC4 synthase promoter region has been associated with increased LTC4 synthase mRNA and a trend toward improved bronchodilatory response to leukotriene modifiers in severe asthmatics. This study will examine mild to moderate asthmatics with the variant gene and evaluate the bronchoprotective response of montelukast in a double-blind, placebo-controlled cross-over fashion. 