In 1973 a survey was conducted among 12 year old children living in a defined area of South Wales. In 1988 the survey was repeated in the same area, again among 12 year old children. Questionnaires were completed for all 965 children in the population sample; peak expiratory flow rates were performed on them all, and repeated (except for five children) after an exercise provocation test. The prevalence of a history of wheeze at any time had increased from 17% to 22%, while that of a history of asthma at any time had increased from 6% to 12%. Current asthma had increased from 4% to 9%, but wheezing in the past year not attributed to asthma had remained at 6%. The exercise provocation tests suggested that both mild and severe asthma had become more common. Increases had also occurred in the frequencies of a history of eczema (from 5% to 16%) and of hay fever (from 9% to 15%). It seems that the prevalence of asthma has risen, and that this cannot be wholly explained by a greater readiness to diagnose the disease.