To test the effectiveness of a house dust mite (HDM) allergen avoidance strategy we conducted a randomized controlled trial in 35 atopic subjects with asthma, aged 13 to 60 living in Sydney — a high HDM allergen environment. After a 3 month run-in period, subjects were randomized to active allergen avoidance treatment (n= 17) or placebo (n= 18) groups and followed for 6 months. The active treatment involved placing impermeable covers over the mattress, pillows and duvet and spraying the remaining bedding, as well as the carpets and furniture, with a tannic acid/acaricidal spray. Subjects kept a daily record of symptoms and peak expiratory flow rates and had 3 monthly assessments of lung function and airway hyperresponsiveness (AHR). Dust samples were collected from the bed, the bedroom floor and the living room floor at 3 monthly intervals and 2 weeks after the treatment. Mean HDM allergen levels at baseline at these sites were, in the active group, 15.5, 9.6 and 10.2μ/g Der p I/g of fine dust, and, in the placebo group 25.7, 11.8 and 6.3μg/g. Two weeks after the allergen avoidance treatment the HDM allergen level in the beds was reduced to 29% of baseline (95% CI 16.50%, P= 0.038 compared with placebo), but was not significantly different at 3 or 6 months. There was also no significant effect of the allergen avoidance treatment on symptom scores, peak flow variability, lung function or AHR (P > 0.1). We conclude that, in a high HDM allergen environment, simple chemical treatment and encasement of bedding is not sufficient to cause a sustained, beneficial reduction in allergen levels. Effective allergen avoidance requires an active strategy to remove allergen reservoirs and control accumulating allergen within the house.