The prevalence of asthma and wheezing in children in Birmingham in 1974-5 was compared with the previous studies in 1968-9 and 1956-7. The frequency of asthma increased over the 20-year period, although the rate of increase had slowed. Asthma was commoner in boys than in girls. Children of the same racial group were more prone to asthma, and probably to other atopic diseases, if born in England than if born in poor tropical countries. Environmental factors in early life profoundly influence the prevalence of asthma in childhood.