Recent studies have associated several chromosomal regions and specific polymorphisms with asthma, atopy and airway hyperresponsiveness. Most of these studies have been cross-sectional which substantially limits their power to analyse genotype/phenotype associations. Asthma is a such a highly variable condition that future genotype/phenotype studies should use longitudinal data. Populations with extensive long-term longitudinal data will facilitate more precise definition of phenotype and allow analysis of asthma's natural history. Longitudinal information also means that each phenotypic attribute can be quantified and considered with respect to age. Collecting longitudinal data prospectively would be extremely expensive and take many years. Using existing longitudinal data sets would clearly be much quicker and more economical, but there are very few suitable data sets to be found. Studying a normal and an asthmatic population is ideal and would provide complementary genotype/phenotype information. This approach is a logical step to using the genotypic information obtained from cross-sectional molecular genetic studies to more critically establish the effect of genotype on phenotype.