Humidity is a decisive limiting factor for house dust mite (HDM) population growth and decreasing humidity may be the control method of choice. This study investigates the effects of portable dehumidifiers on the mite:counts and concentration of the HDM allergen Der p I in the homes in northwest England. Mite counts and Der p I were measured in the paired dust samples collected by vacuuming a 1 m2 area of bedroom carpet, living room carpet, mattress and sofa for 2 min in six houses supplied with the dehumidifier and six control houses, before and 1, 2 and 3 months after the instalment of dehumidifier. Temperature and relative humidity were recorded daily. There was no difference in mite counts in either of the groups throughout the study. Derp I decreased significantly in both groups and in all sampling sites, but no significant differences in the levels of reduction between the groups were found. Condensation was decreased in the dehumidifier group, but the level of indoor humidity capable of retarding mite population growth was not achieved. A single portable dehumidifier placed centrally in the house is unable to decrease indoor humidity to the level capable of retarding mite population growth and decreasing HDM allergens in the type of houses predominantly found in the northwest of England.