Setting: The prevalence of asthma is useful for studying the causes of asthma.
Objective: To ascertain whether there is a relationship between the prevalence and incidence of asthma.
Design: The association between age and the occurrence of asthma was analysed in an epidemiological study of 15,813 persons.
Results: Different conclusions were reached depending on whether the point prevalence, cumulative prevalence or the incidence rates were studied. The relation between the incidence and prevalence of asthma is described in two epidemiological models, and none of the models seem to fit empirical data. Furthermore, it is shown that estimating incidence rates by prospectively measuring the point prevalences may introduce a considerable bias if the reliability of the diagnosis of asthma is around or below an agreement of 99%, which is probably usually the case. Including asthmatic symptoms during the last year in the definition of point prevalence means that there is no simple relation between incidence rates and point prevalences.
Conclusion: The point prevalence may be a biased measure in the study of the etiology of asthma, as there is no simple relationship between the incidence and prevalence of asthma.