Encasings for mattresses, blankets and pillows in combination with mite allergen reduction on the floor have proved effective in reducing bronchial hyperreactivity of mite-allergic children. We studied the effect of combining the use of encasings with specific immunotherapy in comparison to the use of encasings alone (control group). Twenty mite-allergic children (Skin Prick Test, RAST, mean age 10 years) with asthma and high domestic mite allergen exposure ( > 2 microg Der p 1 + f 1/g mattress dust) were included in our study. The concentration of these major allergens as well as specific and nonspecific bronchial hyperreactivity (PC20 FEV1 histamine) were assessed before, 6 months after and 1 year after start of treatment. All children received elongated polytetrafluorethylene (ePTFE; Goretex/Intervent Allergy Bedding System) encasings. A subgroup of eight children were treated additionally by specific immunotherapy with a partially purified mite extract (ALK Scherax) and a maximum dose of 100000 SQ-U Der p + f. Initially both groups were comparable with respect to the parameters assessed. In 80% of children, encasings reduced Der p 1 and Der f 1 concentrations on the mattress to below 3% of the initial values (P < 0.01). The median reduction was 99%. PC20 FEV1 histamine increased from 0.4 to 1.4 mg/dl (median) in the combined group with immunotherapy and encasings (P < 0.05) and remained essentially unchanged in the control group. PC20 FEV1 Der p did not increase significantly in either group. CONCLUSIONS Specific immunotherapy with allergen extracts is an effective adjunct to encasings. Encasings, a key factor for reducing Der p 1 and Der f 1 on the mattress, should be used in combination with other avoidance measures (e.g. removal of carpets) in order to improve allergen-induced bronchial hyperreactivity.