Forty-six asthmatics with verified allergy to the house dust mite, D. pteronyssinus (Dp), participated in a double-blind study comparing the effect of 2 years' hyposensitization with two different Dp extracts. Two groups received either monomethoxypolyethylene glycol modified (mPEG) Dp extract or the corresponding non-modified extract, and a third group acted as controls receiving no injections. Medicine consumption, symptom scores, and peak expiratory flow (PEF) were recorded daily from September to December prior to and after 6 and 18 months of treatment. Changes were calculated choosing changes greater than or equal to 10% as relevant. In addition, patients were asked to give their direct assessment of the clinical effect at the end of the study. After 6 months, there was an improvement in symptoms + medication in 11/14 of Dp-treated, 6/17 of the mPEG-Dp group (P greater than 0.05) and 3/15 of openly treated controls. Few patients had changed in PEF. During the second year, several Dp-treated relapsed and some controls improved. At the end of the study the same improvement rate was seen in all groups. Similarly, the retrospective questionnaire data did not disclose any significant differences between groups after 2 years. In conclusion, hyposensitization with unmodified Dp extract seemed to have a favourable short-term effect on bronchial symptoms + medication in the majority of patients. When mainly on maintenance dose, the beneficial effect was reduced. The mPEG modification of the extract had reduced not only allergenicity but also the clinical effect of equal doses. Changes in medicine and symptom scores only partly correlated to retrospective assessment, thus stressing the problems in this kind of evaluation.