Background: Hypersensitivity to house dust mite allergens is associated with increased asthma morbidity. Asthma severity appears to be related to the degree of mite allergen exposure. Short-term studies suggest that complete avoidance reduces disease severity.Objective: The study was designed to assess the effect of different mattress covers and floor coverings on mite allergen concentrations in the homes of mite-sensitive children with asthma in the city of Melbourne, Australia.Methods: Mite allergen Der p 1 concentration was measured on mattress covers, mattress surfaces, and carpeted and uncarpeted boors in 107 dwellings; and measurement was performed on three occasions over a 5-month period. After the first sampling, all mattress covers and impermeable encasements mere permanently removed.Results: The initial geometric mean concentrations of Der p 1 (micrograms per gram of fine dust) from the surfaces of sheepskin, wool, and cotton mattress coverings were greater than those from the surfaces of impermeable mattress encasements (116, 113, and 19 vs 0.4) (p <0.001); corresponding concentrations on the underlying mattresses were 142, 38, 20, and 0.6, respectively (p <0.05 to 0.001). At the end of the study these mattress surface concentrations were 79, 65, 9.7, and 3.1, respectively. In 24 dwellings an uncarpeted room was adjacent to a carpeted room. At each visit the concentration of Der p 1 in uncarpeted rooms was below the reported threshold for sensitization and significantly less than that in the adjacent carpeted room.Conclusion: In homes of children with asthma, ''asthmogenic'' concentrations of Der p 1 were found on nonencased mattresses and carpeted floors, but the use of impermeable mattress encasements and carpet exclusion were associated with concentrations of Der p 1 below the reported threshold for sensitization.