Background: The indoor levels of mite allergens are known to determine the thresholds of sensitization and asthma exacerbation. However, the method for preventing mite sensitization by reducing the levels of house dust mites (HDMs) is not well established. Objective: We investigated whether mite-blocking bedding encasing made from microfine fibers can prevent infants from being sensitized to HDMs. Methods: Fifty-seven Japanese infants with atopic dermatitis who had high levels of IgE antibodies against either egg white, cow's milk, or soybean (but not against HDMs) were randomly chosen and divided into two groups. Thirty families of atopic infants (group A) were instructed to decrease HDMs by controlling the indoor environment, including bedding cleaning, whereas 27 families receiving the same instructions (group B) were further guided to use the Allerguard encasing for quilts and mattresses of all family members. We repeatedly examined Der p 1 + Der f 1 in the infants' mattresses and anti-Dermatophagoides farinae (Df) IgE in the infants' sera, and we performed skin prick tests with Df extract for 1 year. Results: The mite-blocking encasing markedly reduced the levels of Der p 1 + Der f 1 (3.0 μg/g dust for group A vs 0.77 μg/g dust for B, p < 0.001). It also prevented the increase in serum levels of anti-Df IgE (2.5 U/ml for group A vs 0.7 U/ml for group B, p < 0.05) and positive reactions to skin prick testing with Df extract (63% for group A vs 31% for group B, p < 0.02) over 1 year. Conclusion: The bedding encasement with the mite-blocking fibers was effective for preventing atopic infants from being sensitized to HDMs, and it seems to be beneficial to modern busy housekeepers. (J Allergy Clin Immunol 1998;101:28-32.)