Beds and especially mattresses are important breeding grounds for house dust mites. We investigated whether supposed differences in the microclimate on water, spring and foam mattresses were reflected in the occurrence of house dust mite major allergens (Der f I, Der m I, and Der p I) in mattress dust. Fifty-one mattresses that had been in use for 8–25 months were sampled. Median concentration of total major allergens was 0.5 μg/g dust (range 0–45) with little difference between the three major allergens and between the three types of mattresses. Occurrence of mattresses with concentrations above and below the proposed risk level (2 μg/g) was similar in the mattress groups. However, the allergen concentration was above risk level in 56%, if washable underblankets were not applied, compared with only 21% in the group with such underblankets. In conclusion, mattresses tend to accumulate allergen rather rapidly. Neither heated water mattresses, spring nor foam mattresses seem to be of any advantage for mite allergies, whereas underblankets made of synthetic fibres in a cotton or synthetic cover, commonly used in Denmark, might play a beneficial role in reducing allergen exposure.