Asthma is a common disease caused by a complex interaction between genetic and environmental factors. Asthma afflicts 17 million Americans. In 1999, more than 5000 persons died from asthma. Given the significant mortality and morbidity associated with asthma, it is important to continue to develop new strategies for intervention. Leukotriene antagonists are thought to be the most innovative approach to asthma therapy in 20 years. Despite their demonstrated efficacy, safety and popularity, the leukotriene antagonists are associated with a significant degree of inter-patient variability in response, which can limit their safety, efficacy and cost-effectiveness. Several polymorphisms in leukotriene pathway genes can contribute to variability in response. The project will determine if polymorphisms in genes encoding 5-lipoxygenase, leukotriene A4hydrolase, LTC4 synthase, multi-drug resistance protein 1 (MRP1) and LT1 receptor proteins are determinants of response to montelukast treatment.