Research evidence supports a causative relationship between indoor allergen exposure and the development of asthma. Epidemiologic studies demonstrate that exposure to indoor allergens, particularly house-dust mites and cockroaches and, to a lesser extent, animal allergens and mold, is a risk factor for the development of sensitization and perhaps respiratory symptoms. Sensitization to indoor allergen is clearly a major risk factor for the development of asthma and allergy. There is also epidemiologic and experimental evidence that, in sensitized subjects, higher exposure to indoor allergen causes morbidity. Most, but not all, prospective studies support the causative link between allergen exposure and asthma. Additional evidence comes from the prevention studies, in which reduction in indoor allergen exposure may lead to improvement in symptoms, pulmonary function, and bronchial hyperresponsiveness, and, if practiced, in early years of life, perhaps the development of asthma and allergy.