Asthma is one of the commonest childhood illnesses, and its prevalence and severity may be increasing. Determining whether this is so has been difficult, however, because of the lack of a widely accepted epidemiological definition of asthma and an objective measure of asthma that is applicable to children and is reasonably sensitive and specific. Furthermore, many studies purporting to show an increase in prevalence have used different measures and studied far from comparable populations over the period of the suggested increase.

Several valid studies have now reported a substantial increase in the prevalence of asthma in some countries. In 1991 Robertson and colleagues in Melbourne showed a doubling over 26 years, to 46%, in the prevalence of children with wheeze on one or more occasions before the age of 7; 21- 23% had wheezed in the previous 12 months.1 These findings have been confirmed by Peat and colleagues, who studied children aged 8-10 in two country towns in New South Wales (p 1591).2 They found about a doubling over 10 years, to 23-27%, in the prevalence of children reporting wheeze in the previous 12 months. This is not solely an Australian phenomenon - Anderson et al report a more modest increase over 13 years, to 13%, in …