Over the last 20 yrs our understanding of the relationship of allergens to asthma has changed completely. This is due to several interrelated developments: firstly, it is clear that bronchial hyperreactivity can be increased by allergen exposure and that it is at least in part reversible; secondly, it is now generally accepted that asthma involves inflammation of the bronchi in which eosinophils play an important role; thirdly, there are a large number of studies demonstrating an epidemiological relationship between immediate hypersensitivity to dust mites and asthma; finally, it has been shown that natural exposure to dust mite allergens is very different from bronchial provocation. Together these findings have lead to the view that indoor allergens can play a major role in causing bronchial inflammation with consequent bronchial reactivity and that this is usually not apparent to the patient. Recently three studies in the USA have established that immunoglobulin E (IgE) antibodies to common inhalants can also be important risk factors for acute asthma. In these studies both dust mites and pollens were significant, but cat and cockroach allergy were also important. The results imply that the management of all asthma requires evaluation of the role of immediate hypersensitivity and that treatment of inflammation in the lungs should include both anti-inflammatory drugs and allergen avoidance.