BACKGROUND Effective mite allergen avoidance measures are presumed to reduce airborne allergens yet the quantity in the air is rarely measured. OBJECTIVE To monitor airborne allergen during a placebo-controlled mite allergen avoidance study. METHODS Bedrooms of 56 atopic asthmatic children were randomly allocated to hot washing and encasing covers + acaricide (active regime) or placebo treatment. Dust was collected from the mattress, bedding and carpets; airborne allergen was measured using Casella samplers and dust settling in open Petri dishes. Der p 1, Der p 2 and Fel d 1 were measured. RESULTS After 24 weeks of mite allergen avoidance the Casella air-samplers collected Der p 1 less frequently in active than placebo-treated bedrooms (0 vs. 29%, P<0.05) and Petri dishes in the active group collected less than baseline (0.2 vs. 0.6 ng/day P<0.05). Homes without cats had less cat allergen than cat-owning homes and when actively treated for 24 weeks showed a greater reduction (P = 0.03) in mattress cat allergen than the placebo group. CONCLUSIONS Encasing covers and hot washing of bed linen reduced mite aeroallergen (and mattress cat allergen in the absence of cats). This could mean dual benefits to a patient sensitive to both mite and cat.