Asthma is an increasing clinical problem and allergy to indoor allergens is an important cause. The hypothesis that house dust mite allergen avoidance, by the use of allergen impermeable bedding, improves asthma control will be tested. A double-blind, placebo-controlled, randomised trial will assess over one year both the ability of this approach to improve lung function and quality of life whilst reducing symptoms and prn-beta-agonist use and, in the second six month period, the potential for a reduction in regular prophylactic maintenance therapy. This trial will be conducted in the general practices within the MRC General Practice Research Framework and will provide information on clinical effectiveness and cost effectiveness to guide future healthcare strategies. All adult asthmatics will be randomised with stratification for mite-specific IgE, smoking status and pet exposure. Allergen exposure will be determined at randomisation in all patients and in a 10% sample at six and twelve months