The aim of this study was to assess the ability of various data collected before treatment to predict the therapeutic benefits of hyposensitization. Thirty-one asthmatics were hyposensitized with extract from the house dust mite Dermatophagoides pteronyssinus (Dp) for 2 years, 15 comparable patients served as controls. The treatment extract was either modified by coupling to monomethoxypolyethylene glycol (mPEG) or administered in a diluent containing Al(OH)3. Improvement would be either a greater than or equal to 10-fold increase in bronchial tolerance to Dp or an overall clinical effect judged from questionnaires plus diary cards. Patients improving in bronchial Dp-sensitivity after 1 year had been more sensitive to DP pre-treatment in bronchi and in basophils, and had a lower FEV1 compared with the patients not improving (P less than 0.05). Occurrence of late-phase bronchospasm to pre-treatment Dp-challenge increased the chance of clinical improvement approximately 3-fold (P less than 0.05). A certain mite exposure seems to be a condition of an improvement in symptoms/medication. In patients improving, the median allergen concentration on mattresses was equivalent to 1,000 mites/g compared with less than 250 mites/g in patients showing no clinical improvement (P = 0.1). Information on Dp-specific IgE, IgG, IgG subclasses, Dp-sensitivity in skin, nose and eyes, age, and duration of symptoms did not permit any prediction of therapeutic effect.