The concentration of house-dust mites (Der. spp.) was investigated in 96 Danish dwellings and bedroom air-exchange rates, humidity and temperature was measured contemporaneously in all the dwellings. A positive correlation was found between indoor air humidity and mite concentration, and an inverse correlation was found between house-dust mite concentration and indoor air-exchange. Dwellings of mite-allergic persons had a higher concentration of house-dust mites compared with those of the non-mite-allergic group. House-dust mite concentration was above the proposed limit in 76% of the homes of mite allergic persons and in 48% of the homes of non-sensitive persons. The high proportion of dwellings infested with house-dust mites, when compared with previous measurements, suggests an increased prevalence of house-dust mites in Danish dwellings. The results support the concept that reduced ventilation in dwellings involves a risk of increased house-dust mite exposure.