To investigate whether the rise in allergic disease is explicable on the basis of an increase in the concentrations of allergen and urban air pollutants to which the population has been exposed, we compared the concentrations of grass pollen, sulphur dioxide and black smoke to which two samples of children with previously measured prevalences of hay fever had been exposed, in Cardiff, South Glamorgan. In these studies there had been a 59% increase in the prevalence of hay fever among 12 year old school children between 1973 and 1988. Exposures to grass pollen of the two populations had been no different, but the earlier sample, with the lower prevalence of hay fever, had been exposed to substantially higher concentrations of ambient particulate matter and sulphur dioxide. The rise in prevalence of hay fever, occurring without a rise in grass pollen concentrations, supports the hypothesis that the population has become more susceptible to airborne allergen. This increased susceptibility is unlikely to be a consequence of concomitant exposure to sulphur dioxide and particulate air pollution.