The prevalence of allergic disorders has increased markedly throughout the world during the past three decades. There has been considerable controversy about the extent to which the increase is real, or attributable to increased awareness or better diagnosis. It seems probable that the increased prevalence is real because longitudinal studies have shown a rapid exponential increase in allergic disorders such as asthma. Since there is a marked difference in the occurrence of allergic disorders between Western and less developed countries, as well as between rural and urban areas, it is likely that environmental (‘lifestyle’) factors are aetiologically involved. It also seems probable that contact with microbial antigens, in the form of childhood infections, vaccinations and contact with farm animals, can exert some protection against the later development of allergies. Studies are in progress to determine whether reduction in risk factors or intervention with anti-allergic therapies can modify the long-term outcome in populations at high risk.