We have investigated the effectiveness of house dust-mite (HDM) allergen avoidance through clean room (CR) therapy in patients with atopic dermatitis (AD) having high HDM-specific IgE RAST levels. All patients (N = 30) demonstrated marked improvement in symptom, a long-term remission (mean, 8.4 months), and significant decreases in eosinophil count (p less than 0.01), basophil count (p less than 0.05), serum lactate dehydrogenase activity (p less than 0.01), and HDM-specific IgG levels (p less than 0.05). By contrast, the patients with AD (N = 11), who scored 0 on HDM allergen-specific IgE RAST score underwent "CR" therapy, and the patients with AD (N = 10) having high HDM-specific IgE RAST levels were hospitalized in a common sickroom (nonclean room). All the patients (N = 21) exhibited slight improvement in symptoms and a significant decrease in serum lactate dehydrogenase activity. The period of recurrence after the therapy was brief. The results of measurement of airborne house-dust particles indicated that our "CR" fell within class 10,000. Therefore, it is very likely that the mechanism at work in the CR therapeutic approach is its ability to eliminate fecal pellets of the HDM.