Dehumidifiers (DH) are potentially effective appliances as coadjuvant therapy in the treatment of bronchial asthma caused by sensitization to house dust mites. The aims of this study were to analyze DH tolerance in asthmatic patients, to assess the parasitological effects and to analyze the environmental effects produced by the use of these appliances in the bedrooms of asthmatic patients sensitized to house dust mites. 10 stable asthmatic patients sensitized to house dust mites were studied. DH appliances (CD-300) were installed in their bedrooms. Each patient was given symptom scoring tables and a portable peak expiratory flow (P.E.F.) during a period of 5 months, 1 month before installing the DH and 4 months afterwards. To study the parasitological efficacy of the DHs, we analyzed dust samples from the bedrooms and determined the Der p I, Der f I and Der II allergens by means of a modified ELISA based on monoclonal antibodies. Dust samples were collected before installing the DHs and after they had been working for 2 and 4 months. Dry temperature and relative humidity measurements at three time intervals (7-9, 15-17 and 22-24 h) were carried out. The 1st measurement was done prior to installation of the DHs in the patients' bedrooms and the 2nd and 3rd were achieved 2 and 4 months respectively after the installation. Statistical analysis was done by comparison of paired means. No significant differences were detected in the patients' symptoms nor in the P.E.F. measurements in the course of the study. Decreases in the house dust mite allergens were observed in 4 bedrooms. A significant decrease in relative humidity in the bedrooms of mite asthma patients after use of dehumidifier appliances was observed (p < 0.01). Significant differences between the measurements of the bedrooms with and without DH were detected (p < 0.01). In summary, DHs were well tolerated by stable asthmatic patients, produced a significant decrease in the relative humidity level and showed some parasitological efficacy.