The concentration of house-dust mites (Dermatophagoides pteronyssinus) was investigated in 96 Danish homes with one or more members suffering from asthma. The air-exchange rates, humidity, and temperature were measured in all the homes. A positive correlation was found between indoor air humidity and mite concentration (P < 0.01), and an inverse correlation (P = 0.027) between house-dust mite concentration and indoor air-exchange was found. Homes of mite-allergic persons had a higher concentration of house-dust mites than did those of the non-mite-allergic group. House-dust mite concentration was above the proposed limit of 10 mites/0.1 g dust in 76% of the homes of persons allergic to mites and in 48% of the homes of nonsensitive persons. The high proportion of homes infested with house-dust mites suggests an increased prevalence of house-dust mites in Danish homes. The results support the concept that reduced ventilation in homes involves a risk of increased house-dust mite exposure.