The occurrence of house dust mites (Dermatophagoides spp) was investigated in the homes of 26 patients with atopic dermatitis (AD), 20 patients with psoriasis and 41 randomly selected homes in Arhus, Denmark. The AD patients with moderate to severe eczema had an increased concentration of mites (median 85 mites/0.1 g mattress dust) compared with the controls (median 8 mites/0.1 g mattress dust). The higher exposure to house dust mites corresponded to a relative risk of 4.6 and a clear dose-response relationship between exposure and disease could be demonstrated. Our results illustrate a clear association between moderate to severe atopic dermatitis and increased exposure to house dust mites in the patients' homes, and support the hypothesis that mite antigens could be an aetiological factor in atopic dermatitis.