Sensitization and exposure to house-dust-mite allergens is an important cause of asthma. Standardized, reliable, and reproducible methods for measuring exposure are essential for the assessment of the relationship between exposure, sensitization, and asthma. This study investigated the variability of the house-dust-mite allergen Der p 1 concentration in reservoir dust collected within whole carpets in living rooms and bedrooms. The carpets of nine bedrooms and 11 living rooms were sampled. Each room was divided into 1 m2 areas measured from wall to wall where the carpet was accessible. Reservoir dust samples were collected by vacuuming each 1 m2 area for 2 min. Der p 1 was assayed by a two-site monoclonal-antibodybased immunometric ELISA. Der p 1 was detectable in the carpets of all nine bedrooms and six of the 11 living rooms. Within these 15 rooms, there was a wide range of Der p 1 levels. The smallest range of allergen within a single room was 0.9 μg Der p 1/g dust (0.2 and 1.1 μg/g; 5.5-fold difference), and the largest was 149.2 μg Der p 1/g dust (0.8 and 150 μg/g; 192-fold difference). The mean range of allergen levels in the living rooms was 11.5 μg Der p 1/g of dust, and the mean coefficient of variation of these rooms was 80.2%, illustrating the huge variation of mite allergen levels within each room. The variation within bedrooms was also large, with a mean coefficient of variation value of 88.7%. The coefficient of variation was significantly lower around soft furnishings or beds (57%) than in the rest of the room (89.3%), with the mean difference being 32% (95% CI 2-63%; P=0.04). In conclusion, this study has shown that there is a great variation of Der p 1 levels between areas within a room. No consistent pattern of distribution of mite allergen within a room was found. Der p 1 levels in areas around soft furnishings and beds varied less than the levels in the rest of the room.