There is substantial evidence that morbidity due to asthma is on the increase in western affluent countries.1 The increases in the prevalence of asthma have, however, only been reported in children and young adults, whereas no major increase in the prevalence of asthma has been found in adults. In an Australian study investigating the time trends from 1982 to 1992, a doubling both in the prevalence of wheeze and airway hyperresponsiveness was seen in children.2 In contrast, in a similar survey of adults from 1981 to 1990 the prevalence of recent wheeze increased only in subjects under 30 years of age.3 Moreover, the prevalence of airway hyperresponsiveness had not increased at all, even among atopic adults. The prevalence of asthma and airway hyperresponsiveness was higher in Australian children than in adults, and the severity of airway responsiveness was higher in atopic children than in atopic adults. Likewise, Yunginger and colleagues in the USA performed a study using a population based computer linked medical diagnosis system to identify individual medical records with diagnoses of asthma.4These authors found an increase in the incidence of asthma between 1964 and 1983 which was entirely accounted for by increased incidence rates in children and adolescents aged 1–14 years.

These observations point towards the importance of childhood years for the development of asthma. In fact, Yunginger and colleagues showed that incidence rates were highest in infants of less than one year of age, particularly in boys. Between one and four years of age the incidence rates decreased, but were still much higher than in older age groups.

The development of wheezing illnesses follows a certain pattern over the childhood years. In a prospective, longitudinal, population based survey, the Tucson Children's Respiratory Study in the USA, wheeze was reported at some time …