Pharmacogenetics is the study of how genetic differences influence the variability in patients' responses to therapy, both therapeutic and adverse. Genetic susceptibility and environmental factors both play major roles in the etiology of asthma. The strong familial clustering of asthma has lead to a surge of research into the genetic predisposition of asthma. The aim of the present study is to utilize a double-blinded prospective cohort study to investigate whether genotype-specific effects occur when assessing the duration of protection conferred against exercise-induced bronchoconstriction by regular salmeterol treatment.