Fifty eight patients under 12 years of age, positive to mites (Dermatophagoides pteronyssinus and D. farinae) according to prick, "in vitro" specific IgE and challenge tests, suffering from asthma and rhinitis, were randomly assigned on a double blind basis to receive per os either a biologically standardized extract of mites (active therapy TA = 30 patients) or a saline buffered solution (placebo = 28 patients). Patients took sublingually increasing doses of the solution, followed by maintenance therapy consisting of 15 drops 3 times a week. The results of the trial were assessed after 12 an 18 months, according to the following parameters: symptom scores recorded in diary cards, total and specific IgE levels, total IgG level, IgG1 and IgG4 levels, lymphocytes underpopulations, nasal challenge test, side effects. During the first 18 months, the patients on active therapy had significantly lower scores (p less than 0.001) and clear variations of rhinomanometric parameters (p less than 0.01); IgG also significantly increased. After 12 months, bronchial specific and specific challenge tests showed significantly higher threshold values in comparison to initial values (p less than 0.05). No statistically significative variation was registered in the placebo group.