Several factors have been proposed to account for the increasing prevalence of asthma. It is agreed that genetic changes in populations would be too slow to account for such a rapid change in prevalence. Most information on the effects of environmental exposures on the risk of asthma comes from cross-sectional studies that do not take temporal changes into account. Robust data linking changes in the environment to changes in the prevalence and incidence of asthma over time are still lacking. Changing environmental exposures may not affect disease prevalence immediately. If exposures exert their effect in utero or early in life, changes in the prevalence of asthma may take more than a generation to become apparent.