The epidemiology of asthma in the Danish population is reviewed with reference to research on asthma in general populations, in general/family practice and in hospitals. The prevalence of asthma in Denmark is on the same order as found in national morbidity studies in Britain and the US. The two-peaked prevalence curve suggests that there are two different diseases: childhood asthma, provoked by allergy and temporarily stopping before adulthood, and adult asthma gradually increasing in prevalence until 60 to 70 years of age and due to hyperreactivity. In Denmark, asthma does not appear to be increasing in incidence. With the exception of the very young, seasonal variation is not pronounced and hyperreactivity is thus more dependent on irritants than on seasonally occurring allergens such as pollen. The concomitant prevalence of asthma and allergic rhinitis was similar to rates reported elsewhere. Problems and limitations of sources of data for epidemiologic studies are discussed.