BACKGROUND Asthma severity among mite-sensitized individuals appears to be related to the degree of mite allergen exposure. OBJECTIVE The objective of this study was to assess the long-term effectiveness of mite avoidance measures in the homes of asthmatic children in Melbourne, Australia. METHODS The concentration of house dust mite allergen (Der p 1) was measured on the child's mattress surfaces and bedroom floors in 85 homes on 10 occasions over a 16-month period. After the first three visits, all mattresses were covered with a semipermeable encasement, and carpeted bedroom floors were randomly allocated to regular applications of a placebo or an "anti-mite" shampoo. RESULTS The concentration of Der p 1 recovered was initially high in the carpeted bedrooms (n = 66) (41.1, 95% confidence interval, 30.7 to 55.0 microg Der p 1 per gm) and mattresses (n = 85) (39.6, 27.2 to 57.7). During the initial observation period the concentration of mite allergen fell in the treatment, placebo, and parental control groups. During the seven treatment periods, no differences were seen between the Der p 1 concentrations in the groups using the "anti-mite" shampoo, placebo shampoo, or the parental control group (e.g., at visit 4; 12.6, 8.2 to 19.5; 14.8, 8.6 to 25.1; and 12.0, 8.1 to 17.7 microg/gm, respectively). In contrast, the concentration of Der p 1 in samples from uncarpeted floors and mattress encasements was low (4.1, 2.1 to 8.0 microg/gm and 4.2, 2.6 to 6.5 microg/gm, respectively) and insufficient dust for analysis was frequently obtained from these sites. CONCLUSIONS There was no additional benefit from the use of an "anti-mite" shampoo. The absence of carpets and the use of mattress encasements was a useful long-term strategy for mite allergen avoidance.