The development of monoclonal antibody-based enzyme-linked immunosorbent assay technology for measuring environmental allergen exposure has provided a benchmark for assessing the role of indoor allergens in causing asthma and other allergic diseases. Epidemiological studies from several parts of the world have shown that immunoglobulin E (IgE)-mediated sensitization to indoor allergens (mite, cat, dog and cockroach) is a risk factor for asthma attacks. A dose-response relationship between allergen exposure and sensitization has been demonstrated for mite allergens, and threshold values for exposure levels leading to sensitization or to exacerbations of symptoms have been defined. Comparative studies on airborne allergen levels have made it possible to determine the properties of aeroallergen particles, their concentration in indoor air, and the relationship to clinical symptoms. Together, these studies provide strong evidence that allergen exposure plays a causal role in the development of bronchial hyperreactivity and of the chronic inflammatory responses seen in patients with asthma. Logically, the primary preventive treatment should be allergen avoidance. Through knowledge of indoor allergen levels, both in dust and in the air, different avoidance strategies have been applied to the various indoor allergens, and there is increasing evidence of their clinical efficacy. Monitoring allergen levels in patients' houses should improve their understanding of the role of allergens in asthma and improve compliance with avoidance measures.